Fl0rizla tells me I should continue the blog-chain of the "Best plugins for Open Source Cross Platform apps I use". I'll first tell a bit about the best Firefox plugins.

• Adblock: The most important plugin is the less visible one. It blocks unwanted advertisements from any website. The filters are quite nice as you can block all type of content: javascript, images, iframe,... My list of blocked url's is quite long, it's a good idea to put it online, it might spare you some time making it. You can find it here.
• Tabbrowser Preferences: I also almost forgot this second plugin. But I think it's slowly becoming useless as more and mode of its functionality is moving natively to Firefox. It enables me to set more advanced options for my tabs.
• Web Developer Extension: Magnificent piece of work. If you’re a web developer, this one is a must have. This toolbar enables you to do all kind of analysis of the pages you’re visiting, resize the browser window to common screen sizes like 800×600, edit the CSS of a page and see the effects in real-time, validate HTML/CSS/RSS, and lots lots more. I'm even surprised to see new updates and great new features.
• BetterSearch: Adds thumbnails and other usefull links to my google search results. It's so well done I often forget that it's not built-in into google :-). It can also integrate into MSN Search, Yahoo Search, del.icio.us, A9, Answers.com (web results), AllTheWeb, ask.com (Ask Jeeves), Clusty.com, Vivisimo.com and Simpy.com
• Fetch Text URL: Simple but usefull plugin that lets me select some text and open the link inside that text into the current tab or into a new tab.

Of course these are not all the plugins I use... have a look at https://addons.mozilla.org/ to get some more. On the Thunderbird side the best plugin is Enigmail plugin that enables me to automagically encrypt, sign, decrypt, check GPG signatures. A must have for every geek using GPG and thunderbird. But why do I bother even telling this? Stupid me, you'll probably already use this one. The next one that should tell you something is Philip...