AutoCopy (selected text goes to the clipboard) and IETab (open any tab using embedded IE with one click; if you want, specified URLs will always open in IE) are essential on Windows.

I really like Linkification, which makes anything that looks like an URL or an email address into a link (you can turn it off with one click if it clutters the page too much).

Forecastfox rapidly became indispensable not only to me but to the non-geek household members. No more waiting for the Weather Channel to get around to it, or clicking some bookmark to NOAA or a commercial service.

Mimulus lets you edit'n'save any web page; you'll need to hack the RDF to let it run on FF 2.0.

In haste: more later.

Instead of hacking install.rdf, you might try Nightly Test Tools 'Make All Compatible' function. It's saved me some time, especially on minor version changes.

My "must-have" extensions include:
Adblock
ChatZilla
del.icio.us
DownThemAll!
ForecastFox
GoogleBrowserSync
IETab
Web Developer
X-Ray

Thanks for the tip about Flashblock. Those things always annoy me, and I couldn't figure out how to get rid of them, and Adblock didn't work either for those!

I'll also give cmSiteNavigation a try!

LiveHTTPHeaders. It's the only way to work if you need to figure out something like cookie traffic between your browser and the server.

An even better option than hacking the install.rdf or using the Nightly Tester Tools to automatically hack all of your extensions, you can go into about:config and add a new boolean value of "extensions.checkCompatibility" (set to false)

This has the benefit of not only allowing you to install/use extensions not designed for 2.0, but it marks those extensions as not compatible. I wish I had known about this before I did the Nightly Tester Tool trick because now I have a bunch of extensions, and I'm not sure which ones are still not "compatible"

As for my list of extensions... (I have a lot) A9 SiteInfo, Adblock Plus, All-in-One Sidebar, Attention Recorder, CookieSwap, del.icio.us, FireBug, Forcastfox, foxyTunes, GMarks, Google Bookmarks Button, Google Browser Sync, Google Notebook, Greasemonkey, Image Zoom, Life-of-request info, Mozilla XForms, Performancing, Poster, Restart Firefox, Server Switcher, Stop-or-Reload Button, Update Notifier, and Web Developer

LiveHttpHeaders, as recommended before.

DocumentMap (https://addons.mozilla.org/firefox/475/), for displaying the h* structure of HTML documents, essentially providing a nice TOC.

LinkWidgets (https://addons.mozilla.org/firefox/2933), an alternative to cmSiteNavigation Toolbar, but taking less screen estate.

A few more, as promised:

Gcache and Wayback to let me follow broken links through the Google cache and the Wayback Machine respectively.

User Agent Switcher, for fooling stupid sites (though I more often use IEtab these days).

TinyURL Creator provides a nice interface to the TinyURL service.

All of these except User Agent Switcher require the about:config trick.

You forgot TinyMenu, Fission and FilterSet G. Updater (For AdBlock)