If Google decides to rebrand Firefox as GBrowser, I think it's quite likely they will try and use XUL (1) as their 'rich client' GUI toolkit. That, or Java (e.g. Swing or SWT) -- as they've been hiring quite a few Java geeks lately (2).

(1) http://www.xulplanet.com/

(2) As rumor has it, I couldn't find the article I saw this in. . .