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How can I run a piace of javacript code on a certain website automatically? [duplicate]

I've written scripts to filter out threads and posts in a programming forum by author.
But you need to post the code into the web console for each page individually.
I was wondering if it were possible and if so how (using either firefox or chrome), to set your browser up to run a specific script upon loading a specific site? Is it possible as an extension or plugin?

You can use Tampermonkey for Google Chrome
Tampermonkey is the most popular userscript manager for Google Chrome.
Greasemonkey for Firefox:
Allows you to customize the way a web page displays or behaves, by using small bits of JavaScript.

You can use dotjs.
dotjs is a Google Chrome extension that executes JavaScript files in ~/.js based on their filename.
If you navigate to http://www.google.com/, dotjs will execute ~/.js/google.com.js.
This makes it super easy to spruce up your favorite pages using JavaScript.

Does pdf.js execute javascript contained in a pdf file?

It appears that PDF has, for lack of a better term the "feature" to contain javascript, but luckily it seems there is a change that only some software implements support for javascript
Since web browsers deal already with the issue to handle tons of untrusted input, and hence do some sort of sandboxing, I rather appreciate to open pdf documents inside the browser sandbox via pdf.js. However I wonder if pdf.js will implement the "javascript contained in pdf file misfeature"?

pdf.js still does not support embedded javascript
There are some other implementations (e.g. https://www.pdftron.com/webviewer/demo/pdf-forms) but most of them have some limitations unless you are looking for simple field/form calculations

Update 2022: Since Summer 2021, pdf.js (and by extension, my library ngx-extended-pdf-viewer) uses Quick.js to run JavaScript in a sandbox. You've already mentioned that the browser runs JavaScript in a sandbox, so this is a sandbox running in a sandbox. The Mozilla team didn't want to give the JavaScript code embedded in the PDF file access to browser resources.
If you're using Firefox as a PDF viewer, you're using a slightly different version of pdf.js. It runs the JavaScript code in the same sandbox that runs Firefox WebExtensions.
Here's an article written by the authors of the JavaScript implementation: https://hacks.mozilla.org/2021/10/implementing-form-filling-and-accessibility-in-the-firefox-pdf-viewer/

Ability to load init script in Firefox?

I have a long time stick with Conkeror as my default web browser and get used to configuring/adding new features to my browser using js code with all the XUL Api through the .conkerorrc file. I'm migrating to Firefox since it has better support and is actively maintained by Mozilla. However, one of the feature that I've been missing so much is the dot file, which I can easily configure anything that I like, back up all of them through git and eval the code directly (using Mozrepl) while I'm coding to see the result.
Is there any way that I can inject/execute a sciprt on Firefox startup, for example ~/.firefox/index.js?

There is no functionality in stock Firefox to execute JavaScript code supplied by the user at startup. Functionality like this has been something that has been requested of Firefox since 2006-04-02.
It is trivial to write an add-on in any of the different Firefox add-on types (XUL/Overlay, Restartless/Bootstrap, Add-on SDK, or WebExtensions) which would run whatever JavaScript you desire upon Firefox startup. This could be done to either run code that was included in the add-on (simple), or that runs the JavaScript contained in a file that is loaded from a location external to the add-on (more complex). Which add-on type you used to implemented this would impact which interfaces you had available within the code you write. One drawback of writing your own extension which runs code included in the add-on is that in order to use it with a release, or beta version of Firefox is that you would need to have it signed by Mozilla. While this is a quick and easy process, it does add some additional overhead to the development/test cycle.
You have not specified any of the firefox-addon tags in your question. In addition, you have not described the functionality you desire, except as generalities. It also does not appear to be the intent of your question to ask how you would implement such an add-on. Given those and the fact that there are already multiple add-ons that implement the functionality of running arbitrary JavaScript (including XUL) code supplied by the user, I am not going to supply code here which performs this function.
However, if you are interested in using an already existing add-on, here are a few options:
userChromeJS: This extension was derived from the code originally provided as an example of how to implement the functionality requested in bug 332529. Its first feature listed is: "Complete chrome customization is possible by running custom javascript code or overlaying chrome with .xul overlays." This is an Overlay based add-on with which you can use XUL. This sounds like the functionality what you are interested in obtaining.
uc: "A userChromeJS clone with built-in subscript/overlay loader capability."
Greasemonkey: "Customize the way a web page displays or behaves, by using small bits of JavaScript." This is a commonly used add-on which permits writing more complex JavaScript code. The code is executed in a sandbox, not in the scope of an extension. This is done for security reasons.
Custom Style Script (Inject desired CSS or JS): "Add Custom JavaScript Codes or Styles (CSS) to an specific page or all pages."

Create Internet Explorer extension changing content of a webpage

I'd like to develop an Internet Explorer extension that changes the content of a specific webpage, like content scripts in Google Chrome. (eg. when I go to the website Google and I search for "car" I'd like to have a div created on the page with the word "car" inside).
I've been looking on SO but haven't found clear evidences if this kind of thing is possible.
What I'm specifically looking for is a clear tutorial or some example of how to do this.
Bruno,

That is exactly what GreaseMonkey for IE is made for http://www.gm4ie.com/

I have answered a similar question, but the answer is in C#. It describes how to create a fully working project to accomplish exactly what you asked: changing page contents!
How to get started with developing Internet Explorer extensions?
It covers other topics too:
running javascripts from the addin
how to register IE addins
saving data (addin configurations)

You can read more about Internet Explorer Extension from MSDN:
http://msdn.microsoft.com/en-us/library/aa753587(v=vs.85).aspx
Extensions in IE are not created using web technologies, they are created using C++/.NET. Such as ActiveX

Similar to "GreaseMonkey for IE" is Trixie:
Trixie is to Internet Explorer as Greasemonkey is to Firefox. It lets you remix the Web via scripts. You may do this to either make it more readable, fix bugs or to even add little features to make the site more usable to you. Trixie by itself does none of this. It is just a plugin for Internet Explorer that enables executing chunks of JavaScript code and thus lets you use the Web the way you want to use it.

This is one of the best I found. http://www.enhanceie.com/ie/dev.asp It has give sample scripts as well. Which will help you.
Another one is open source and stable version : firebreath
same kind question is also asked How to get started with developing Internet Explorer extensions?

This is a working example of manipulating the DOM from microsoft. This is a BHO development with MS Visual Studio. Check it out: http://msdn.microsoft.com/en-us/library/bb250489%28v=vs.85%29.aspx

How to create a Firefox add-on using Objective-C on Mac OS X?

More precisely my goal is to create an add-on (or plug-in?) which is able to communicate with my main Cocoa application using something like the NSDistributedNotificationCenter. I need to be able to inject JavaScript code into the current webpage and get return values from the JS calls when my add-on receives the request to do so by my main application. Then I need to pass the return values back to my main application for processing.
Alternatively if there is a simple way to call JS in the active Firefox webpage and get return values that would also do the job.
If you want more info on why I want to do this, you can look at my other question: How to send JavaScript code to IE using C# (.Net 3.5), run it, then get a string return value from the JS code?
Note that I'm not only interested in knowing how to make a Firefox add-on but also in everything I talked about above. For example, how to inject JS into the active webpage, etc.
I'd like guidance on what technologies to use, tutorials and sample code if possible. The best would be a sample Xcode project but I'm not counting on this :P
Thanks in advance!
N.B: I'm working on 10.4.

You could try using C-Types with FF, which is a regular dll being called by Javascript in your addon, this is WAY better approach that using XPCOM, because if the Interfaces you use in there can change in each FF version, indeed you will have to do multiple dlls each for your addon supported FF versions
Go here my friend->
https://developer.mozilla.org/en-US/docs/Mozilla/js-ctypes/Using_js-ctypes

First learn how to write add-ons for Firefox. Adding Objective-C code afterwards is the easy part.

I know there is source available to the OSX FF Plugins for displaying download progress over the Dock Icon (http://github.com/vasi/firefox-dock-progress) and the "pdf plugin" that allows FF to do in-line PDFs is on Google Code.