My experience of Firefox (I have the latest version auto-updated via the ß channel, and FF is set to auto-feedback to the developers) is that FF is usually rock-solid, and ß updates are released rapidly if the developers become aware of any problems in a release. Otherwise, if anything seems to go "wrong" with FF, it is invariably something peripheral to the application. Typically, one or more of the add-ons have not been able to be updated to keep in step with the latest ß version.
As an example, I have had some recurring and annoying problems with tabs for months, and after the latest ß update they got really bad. In desperation, I disabled half of the add-ons (including all the add-ons that affected tabs) and then progressively re-enabled them. The problems entirely disappeared, and I have no idea why.
As for speed, FF seems to be very fast. FF occupies little CPU, and because I run cache from RAM it is made even faster - or at least as fast as it could probably be. As I write this, FF CPU usage is flickering between 0-2% (max). I have 7 tabs open, 3 of which are permanently pinned/open - the Add-ons page, Gmail, and Google Reader.
Bandwidth doesn't seem to be a problem. I normally use a wifi access (n-type 802.11n) to connect to the router. On the odd-occasion I have tried a cable connection to the router, the response time seems pretty much the same either way. I haven't properly tested the current configuration's up/down load speed limits via wifi and cable.