My Firefox 3 beta goes gray annoyingly often. Last night, at BLUG, Kevin Fenzi said that it's a bug related to SQLite, and the workaround is to uncheck the tracking of problem sites, in the Preferences menu.
I haven't decided whether to give that a try, or just wait for the upgrade.
I also notice that someone, on the Google Group used to track Gears tasks inside Google, claims to have fixed the Gears/Firefox 3 incompatibility. I'll believe that one when it shows up. I like the idea of Gears, so it's a pity I can't use it now.