My recent experience with Firefox's speed
Then recently I upgraded from my personally compiled vintage 2006 Firefox (which was more or less some 3.0 alpha or beta version, built from the then current CVS source code) to a current Firefox 3.6. The result of this has given me a whole new appreciation of this issue, because my new Firefox is, shall we say, not speedy. In fact it's frequently decidedly pokey, with clear pauses every so often when I want it to do things like open new windows, follow links, or even scroll text. Many things seem to stutter or otherwise not work anywhere near as well as my old Firefox did.
I'm not sure what the cause of this bad performance is but my suspicion is on Firefox's new sqlite-based history database, which is known to have problems on Linux to start with. I keep a perpetual browsing history, so I have a very large history database (my home database sqlite file is 185 MBytes); I can believe that even checking it is kind of slow (especially if it involves actual disk IO). By contrast, Firefox's old history database basically kept everything in memory and seems to have worked fine for me despite the size.
(The obvious experiment is to temporarily throw away my history or drastically reduce its size, but that has certain downsides. I keep my large history because I find it very useful, after all.)
Sidebar: why I finally upgraded my old Firefox
The short answer is 'it seemed about time'. Two things pushed me over
the edge; first, my old Firefox was crashing fairly frequently when I
browsed LiveJournal, and second, it looked increasingly like I was going
to have to give up
xfs on Fedora because using xfs crashes the X
server and the bug
did not look like it was going to get fixed. My main reason for still
using my old Firefox was that it was the last Firefox version that could
use the fonts I like, but it needed xfs for other fonts; if I was going
to have to give up xfs, I was effectively going to have to give it up