STEVEN GOODHEART  Same experience here on my iMac G5.....according to Activity Monitor, FF uses 30 to 60% of my CPU and often jumps to 80%...typically about 40% with 5 or 6 tabs opens, way more if 10 or more tabs are open on demanding sites.....Safari, right now, is using about 6 to 7% with 5 windows open....my experience has been that, on the whole, FF uses way more resources than Safari and therefore in most cases, is slower than Safari.

I really, really like the add-ons and customizability of FF, but until I get a faster machine, it's just too much of a drag on overall performance when I have other resource hogs open too.

In the meantime, I have found that with SafariStand, Saft, Safari Tabs, and Keywurl, I have most of the "can't live without" features I admire on FF, not to mention the ability to use the Services menu, which really unlocks the power of programs like DevonThink Pro, MacJournal, NoteTaker, and Scrivener.  
(Version 3.0.1)

SMTIPS  bwahahaa...you think you need a faster machine than an iMac G5 to run a web browser? Geeez...if Firefox is going THAT slow on a G5 they need to take it back to the drawing board til they get the garbage FIXED! :)  
(Version 3.0.1)

ARKAYN  On my iMac it usually only goes up to 15% when I am loading pages and then drops down to 2% when idle.

Beta 4 did have a memory leak that did what you are describing.  
(Version 3.0.1)

STEVEN GOODHEART  I'm using 3.0.1, so I doubt if it's a memory leak like there was in Beta 4. In fairness to FF, I have 25 Add-ons installed, and many of them are really powerful and feature rich, and I'm sure that they must add to the overall resource load:

Addblock Plus, All-in-one Sidebar, BlueOrganizer, Clipmarks, Customize Google, Define Word, Digg Firefox Extension, DownThemAll!. Fast Dial, Feed Sidebar, Firefox Search Sidebar, Firefox Showcase, Google Notebook, Interclue, OPML Support, PDF Download, Quartz PDF Plugin, Rewind/Fastforward Buttons, Sage-Too, Save Line in Folder, Scrapbook, Session Manager, SlimSeach, StumbleUpon, Surf Canyon Search Engine Assistant, Tab Mix Plus, Taboo, Toolbar Buttons URL Fixer...whew!

Yeah, that's probably "too many" :) but each of these really add-ons rock and add to the utility and ease of use of FF. I'd be curious to know if someone who is getting a much lower CPU load many feature-heavy Add-ons.

And look, this speed thing is all "relative," right? It's not like FF brings my machine to its knees or something! But if I have 6 to 10 other programs open that are also resource heavy, and I'm working on images or sound files, I definitely see the spinning beach ball more than I'd like, and things are noticeably slower when I use FF versus Safari -- but again, this is relative. When I think about what I'm doing, and the program horsepower I'm dealing with, it's pretty amazing. Still, when I can eventually afford to go from a 1.8 Ghz G5 iMac to a 3.0.6 Ghz Intel Core Duo iMac, I sure am going to do it! :)  
(Version 3.0.1)

UMAROMC  There's NO WAY that Firefox is eating that many CPU cycles without some add-on being responsible.  
(Version 3.0.1)