Mozilla is not the most popular browser by a long way, but I do believe it is the best. It's not the fastest browser, but I wouldn't call it slow.
What it has is the most accurate and reliable rendering engine available, Gekko, and a host of features unmatched by any other browser – especially if you design Web pages.
I won't go into the advantages of its mail facility other than to say the word 'security'!
Web Developer Toolbar
Converts between GET and POST. Displays all form details on the page - names, ids, values. Enable/Disable auto completion. Show Passwords - be very careful with this one as it turns those ****s into the letters they represent and exposes your passwords on a page.
Displays image dimensions in pixels, shows broken images, disables images (removes them from the page flow), makes images invisible (keeps the page flow but hides the images). Indicates images that don't have ALT or TITLE values. Replaces images with ALT values (text).
Shows Cookie information for the page (if any). HTTP Header information. General page information. Page bytesize/download speed report for different connections.
Draws a visible outline around block level elements <divs>, links without titles, tables or any custom element you choose. Great for examining layout structures.
Sets the browser window to popular screen sizes 640 x 480, 800 x 600, 1024 x 768 – or any custom size you choose.
Sends your current page to various validators to check HTML, CSS, Links, Section 508 Accessibility, WAI Accessibility.
Shows the source for the current page in a tabbed window.
This extension is an absolute MUST. If you don't use Mozilla already, it is worth using for this facility alone! It can be switched on and off in the View/Show/Hide menu but I would like to be able to disable the Show Password feature with a master password for security reasons.
Similar to the Google bar for Windows IE. Provides lots of standard and special search options from the menu bar or from a right button contextual menu. As this is not officially affiliated with Google, the Google Ranking feature is missing but is nevertheless, very useful.
Checky is the big daddy of all syntax checkers and will whip your page off to more than 45 different online validators to check just about every conceivable aspect of the page - HTML/XHTML, CSS, Accessibility, Links, Search Engine friendliness, backward compatibility viewers. Options are available in a contextual menu.
User Agent Switcher
Lets you spoof the user agent of other browsers - IE6, Netscape 4.6, Opera 7.21.
Many sites will not let you in if you don't use their 'approved' browsers - especially banking sites. Opera and Safari have this feature and with this extension, you will have it in Mozilla too. By switching my Mozilla user agent to that of IE6, I was able to get into my online banking account with Mozilla, something I've never been able to do before.
I'm not sure about the situation with this one, it's not on the Extension Room page and doesn't seem to have been updated since Nov 2002.
CasCadeS adds CSS editing functionality to the Mozilla 'Composer' Web Page editor and does so in a very user-friendly way but it seems to be in a hiatus and doesn't work with Mozilla 1.6 as far as I can tell. Pity!
There are many more extensions which might or might not be of use to you. Some are specific to Firebird or Thunderbird and some relate to mail rather than Web. Mozilla 1.6 runs on Windows (32), MacOSX and Linux.