I am a long time Firefox user and advocate but I have recently decided to move to Google Chrome as my main (non development) browser. 

Excessive memory usage has been a longstanding problem with Firefox.  Up until now, i have endured the memory penalty for the sake of a number of near indispensable Firefox add-ins (the multi-machine bookmark synch functionality in XMarks is top of this list).   I have, however, found that recently, perhaps as a result of the ever increasing number of media rich and JavaScript heavy websites, that the memory usage in Firefox has become completely unacceptable. 

Here is my task manager after using Firefox 3.0.11 during a typical 8 hour work day (read blogs, reddit, ycombinator, watched a couple of Mix09 videos, etc),

To add insult to injury, if I close all tabs apart from one, Firefox only re-claims approx. 20mb of memory.  I know that the memory wastage might be down to the extensions that I have installed but frankly I’m past caring.  If this is the case, Firefox should limit the amount of memory available to each extension. 

I have trailed Chrome over the last few days and I like it a lot.  Its lightning fast and the process per tab architecture makes it more stable than either IE or Firefox (it also means that if I close down a tab, the memory used by that tab is immediately reclaimed).  I have found website compatibility to be good - its only had a problem with a couple of heavy flash and JS sites.  The add-in architecture is still under development, but its only a question of time before the add-in ecosystem rivals that of Firefox.

I will still use Firefox as my main development browser.  Firebug is actually indispensable when it comes to web development.

I know nothing about the internal architecture of Firefox, but I’m guessing that as excessive memory usage has been such a longstanding problem there is something inherent in the architecture that makes it nearly impossible to solve without a major re-write.