I upgraded to the latest Firefox package available through Synaptic Package Manager (package version 1.0.2-0ubuntu5.4 in hoary-security) — and therefore the Ubuntu package repositiories — just then, and it’s been segfaulting on startup ever since.
josh@joah:~$ firefox --verbose
FIREFOX_DSP=esddsp
APPLICATION_ID=firefox
CMDLINE_DISPLAY=
DISPLAY=:0.0
REMOTE=0
TRY_USE_EXIST=0
OPTIONS=
DEBUG=0
DEBUGGER=
Running: /usr/lib/mozilla-firefox/firefox-bin -a firefox -remote 'ping()'
PING_STATUS=2
Cleaning user profile
Running: esddsp /usr/lib/mozilla-firefox/firefox-bin -a firefox
Segmentation fault
It’s not a huge problem for me, as I’ve recently installed Opera 8.01 on this machine and have absolutely fallen in love with it (but haven’t been using it as my primary browser because all my settings, cookies, etc. are stored in Firefox), and I’ve got Firefox on three other desktops should I need to test a website in it, but it’s a bit annoying.
Hoping whatever’s wrong with the package will get fixed relatively quickly…
UPDATE: Curiously, Firefox appears to launch without any problems whatsoever when a URL is specified (e.g. firefox http://www.joahua.com/blog/ as a launch command). So, now it merely belongs in the realm of irritations as the bug can be circumvented with minimal effort.
A few moments later: Oh, no, apparently not. If one opens a new tab, Firefox decides to crash once again. Cruel world. *wanders back to Opera*

Bug alert for last release! Downgrade:
sudo apt-get install mozilla-firefox=1.0.2-0ubuntu5 mozilla-firefox-gnome-support=1.0.2-0ubuntu5
Ah, excellent! Thanks for that, I wasn’t aware you could downgrade that easily.