6th

To set multiple home pages (which open in separate tabs) in Firefox, separate the links by a pipe “|”.

To set multiple home pages (which open in separate tabs) in Firefox, separate the links by a pipe “|”.
Update: It does not work on Win 2K after all, so if you just wish to download (but not install) then you could use this article.
Update 2: It can be made to work using this.
Though Google Chrome runs on Win 2K SP4 Win XP SP2 and above, the download link only allows XP and Vista users to download. To download on a Win 2K system, you would need to change the user-agent of the browser.
On Firefox, go to about:config, create a new string entry general.useragent.override and set it to this (or something similar).
Mozilla/5.0 (Windows; U; Windows NT 6.0; ru; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1
DailyKos, a popular political blog, displays this warning if you use Adblock Plus.

Normal users don’t see this message as the HTML element is hidden via CSS.
http://www.dailykos.com/css/adblocker.blogads.css
Ironically, Adblock Plus “unblocks” this message by blocking the CSS file (since it contains the pattern .blogads.)
This Firefox extension adds a right-click context menu option “Pronounce” to every word.
Popular Firefox extensions get installed well in excess of a million systems. A lot of them also request information from various sites for their operation.
That makes them potential DDoS agents. What if a massively popular extension turns rogue, and releases an update which requests info from the target site ? Requests from a million systems, most of them not even aware of it, could easily cripple all but the biggest sites.
This is perhaps one of the reasons why Firefox phones home daily to disable rogue extensions.
A Boss Key, a keyboard shortcut to quickly hide a computer game and display a fake work screen, used to be a fairly regular feature on DOS based games.

Panic is a similar Firefox extension which closes all open tabs, and replaces them with a Google search for “Increasing Workplace Productivity”.
Google has a neat transliteration tool, which allows Hindi (or other Indian scripts) to be typed using the English alphabet. For instance “mera” is transliterated to “मेरा” (Hindi font support needed)
I’ve used that to write gIndic, a Firefox extension which provides transliteration service on any website. Like the Google service it supports Hindi, Tamil, Telugu, Kannada and Malayalam. It works on both Firefox 2 and 3.
You should now see the gIndic icon - the letter “A”. To start transliterating, click on the “A” icon and choose your language. Click on “A” icon and choose the “Default” option when you want to type in English again.

Type a word and hit a space to translate it. It works on input and textarea tags.
Firefox seems to have a neat undocumented function find ().
When called without parameters - find (), it opens the find dialog box. With a parameter - find (‘firefox’), it searches and focuses on the first found string.
I’ve been trying out Prism, the platform for running Web Applications directly on the Desktop.
I’ve been looking for something like this, my Firefox installation gets too crowded with plugins, leaking memory and decreasing responsiveness. A platform like Prism allows me to create a trimmed down Firefox for Gmail, Netvibes, and other Web-applications.
The trouble is that while it is low on the features, it is not so much on the resource usage. Moments after starting Netvibes, my RAM touched 50Mb, consistent with what a resource / plugin / feature intensive Firefox would consume.
Anyway this explanation sums it all pretty well.