I have had to stop using Firefox. I have researched in Mozilla and other Ubuntu Forums. I'm running current FF, U11.04, and I have run the gamut disabling add-ons etc. I have Gmail and Google Reader as AppTabs or whatever they are called.
Within a few hours, and often within an hour, my whole system slows to a stop where a hard reboot is the only recovery.
There are "hints" on the interwebs that this is a gmail problem but nothing definitive. No problem in Chrome.