Wouldn't the version problem vanish if they had a separate version number for extensions?
E.g.: (using abi for simplicity; pretend it's api if it makes you feel better)
Extensions compatible with ext abi version 1 work with Firefox versions > 4 and <= 6
Likewise, ext abi version 2 work with Firefox version > 6 <= 7
Because it's the browser that will keep track of which abi version works with itself, the burden is lifted from the extension somewhat. They'll still have to make changes when there's a new extension abi break, but for multiple browser versions they otherwise don't have to change anything.
The 3D acceleration you are talking about is only used for WebGL, which is hardly used on the web. So it's not the cause of your memory leaks, unless you were specifically on some webpage that uses 3D features, which i doubt.
That's good.Also, starting with Firefox 10 they will default extensions to be compatible rather than incompatible.
Chrome annoys the hell out of me! Every time I have tried to use it. Its the BTRFS of the browsers! Good on paper but never delivering the thing!
Personally I don't really care too much about memory usage, all my systems have at least 4 gigs of ram and I never get close to running out.