Edit: Solved! Thanks to the comments of several people, I was able to find the magic sequence of steps necessary to fix my specific situation:

1. Within firebug, go to Tools/Add-ons, and uninstall firebug. (without this it looks like you are updating, but you are still stuck with the old previously installed plug-in).
2. Install firebug from apt-get, aptitud, synaptic or what have you, since it is true it is right there in the universe repository.
3. Forget Firefox 2, Firefox 3 rocks!

[Skip directly to the solution on how to magically regain your old Firefox 2 with all its plugin info intact after upgrading to Ubuntu 8.04 "Hoary Heron".]

Well, this has nothing to do with Drupal, yet everything to do with Drupal. And it's not Drupal that comes to the rescue (it's just soooo cool to say that), but the sanity, calm, and above all intelligence that is contagious in the community that saves the day. That is, one gets so accustomed to having to get to the bottom of things to get them to work just right, that a little thing like losing all your plugins from your workhorse simply puts a quizzical smile on your face, instead of the hair tearing hysterics that would be the case for (ugh) Ruby on Rails developers, poor souls.

Anyway, until most of our sites are upgraded to Drupal 6.x, we still spend most of our time in the Drupal 5.x environment (and probably will for some time to come). Which means we cannot avail ourselves of Moshe Weitzman's amazing "Firebug for Drupal theming" Theme developer, even though the rest of the great devel project (same link) is probably even more a part of the Drupal web app developer's stock in trade.

Which means that we spend quite a few hours per week (sometimes per day) in Firebug.

So... I know a lot of us in the Drupal Community use Ubuntu desktop boxes for development. I swear by it.

So... When the exciting notice appeared "Upgrade available 8.04", I hit that button with (well deserved) confidence.

Since most of the rest of the Ubuntu using world was doing the same, it took about a day for the files to download, but it was a flawless upgrade.

Until I finished the upgrade early this Saturday morning and rubbing my hands together sat down to work on a custom module to compare nodes side by side, fired up firefox and... whaaaa?

I was informed that none of my add-ons (Firebug, Web Developer, Selenium, Wizz, even del.icio.us) would work in Firefox 3 beta, which the upgrader had installed without consulting me (it did ask about keeping php.ini and the existing apache 2 server configuration, for which I am eternally grateful), but that it would check and see if upgrades were available. Cool. But no upgrades available, I am afraid. And I was presented with a very vanilla looking browser screen indeed.

Thousands of thoughts raced through my mind. I did find a link to Firebug 1.1 that would work on Firefox 3: http://fireclipse.xucia.com/ But to download it you had to go to www.getfirebug.com, ... which was down!

So, I thought, I will look at synaptic, to see if I can install Firefox 2 alongside the new 3 beta version, or replace it, or maybe put in one of the other Mozilla browsers that supports Firebug... So I fired up Synaptic, and lo and behold, I saw that Firefox 2 was installed alongside Firefox 3. Oh happy day!

So I went into the Applications menu (Applications / Internet / Firefox 2 Web Browser). And fired Firefox 2 up. Oh happy day, callou, callay!

But wait... the browser came up OK, but without any of the plugins... Oh why, ohy why hadn't the Ubuntu engineers left the plugins to roost in Firefox 2, and left the Firefox 3 all blankity blank...???

Well, you fool, you, you doubting Thomas, it's because they both share the same configuration directory, and the problem, it turns out, has nothing to do with Ubuntu... but rather because of the fact that the spanking new Firefox 3 doesn't use the old extensions.rdf file anymore for keeping track of plugins and so forth, but rather sqlite. Anyway, how do I know this? Because I resigned myself to either figuring out some way to transfer the plugin info from Firefox 3, to whom it was useless, or, to simply installing the plugins again. It was when I was installing Firebug the old and trusty onto the blank looking Firefox 2 browser, I get this error:

"installLocation has no properties" error in nsExtensionManager.js during install/update of extensions

This turned the quizzical smile into something of a jaw dropped gape. Never mind, a quick Microhoo on this error, and I found saviours: mine and yours: this link to the mozilla bug queue: Bug #65609 in Mozilla Firefox, at https://bugs.launchpad.net/firefox/+bug/65609 .

Very interesting discussion, but the short of it is:

Solution

Go right into the promiscuous ~/.mozilla/firefox/{weird profile name}/ directory, and rename the file extensions.rdf to extensions.rdf.old, or whatever. As the above bug report page explains, if you then close all mozilla browsers running, then bring up Firefox 2, ... it will create a new, healthy extensions.rdf and... lo and behold, oh frabjous day! Firefox 2 comes up with all your plugins flashing and smiling. Good old Alt-Q to waste some nice time on my RSS feeds, good old Firebug, esteemed Selenium, the old ugly blue de.licio.us buttons, both of them, there intact.

What a relief.

Until I accidentally ran Firefox 3 beta again, instead of Firefox 2, and everything went back to the way it was before.

"Beware the Jabberwock, my son!
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch!"

Well, as you probably guessed it, repeating the renaming exercise of extensions.rdf worked a charm, and all was well.

I punished the Firefox 3 beta browser by putting a Debian icon on it, to avoid future mishaps.

Now, let's get to work...