See nothing that requires ''more speed'' (6x is claimed in the update pop-up for 4.0) when browsing with either FF3.6.18 or 4.01. The 5 'sbeen offered since 2 days ago on Linux. When the word finally comes that having 30 tabs open and in minimized state it not gobbling RAM AND CPU cycles, it could be time for a look. Till then give it a few months to get all your extensions and add-ons to update and remain collaborating... counting 51 on me laptops right now, including 8 dictionaries. Still preferring TabGroups over the Ctrl-Shift-E before one can see what's open. CoolPreview+FoxTab do an equal good job and most irritating the Firefox Button overlays other elements, so one has to minimize it before being able to for instance get to Dash on Linux.

Just an opinion :O

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No add-ons rejected for me.

It is looking also the same under Windows.

Just FF asked to be updated, and I said: sure, go ahead ;-)

Not using any of those fancy add-ons ;-) I am a basic surfer...

Took the plunge and added a second install of 5, which I made to go into it's own program folder, so as not to loose a known a good/stable working 3.6.18 set. Of the 51 add-ons mentioned above 24 proofed to not have been kept in an outright compatible state. After much searching found replacement add-ons for half a dozen not maintained. Another half dozen would not auto-update but there were new versions, so forced those to come across in a painful, disable, uninstall, install cycle, restarting restarting restarting, such as for IE Tab Plus to run in IE rendering mode, which made the ''Quick Restart" add on, no longer compatible a must to have a substitute for, which was found with "Restart Firefox". Now I'm at 9 left that have been left disabled else they break something or cause the page load to never finish, sucking 40% CPU time of 1 core. Did gain a few new:

Textarea Cache, will remember what you were writing through the last keystroke before a FF crash or restart [secure as you can specify for which sites you dont want it to remember anything.
Tab Scope, gives tab thumbnail images of size you can define when hovering over tabs. Fast and zoomable.

Lost one of me much liked... FavLoc and could not get it to work even when editing and repacking the xpi. It wont save favorite download store locations.

But, it does appear crispier and faster and TabGroups Manager is working better than before and me french spellchecking is now ''Classique & Réform 1990 :P

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PS, the Firefox button and the open/close/min buttons still half overlay the forward/backword/refresh and zoom +/-/Split Browser buttons [latter allows to have multiple pages in a browser view]