Note: I am on the Mobile Firefox development team.
Load time is definitely an issue on Maemo. The built-in MicroB browser uses "faststart" which means that it starts a process when the device starts up, and that process stays in memory even after you close the browser. Firefox 1.1 doesn't use faststart, but we and Nokia are working on it for version 2 which will be the default MeeGo browser.
The "fennec" process running after you close the window is a bug, and one that I don't think we've seen before. If you'd like to help us solve it, you can report it here or to bugzilla.mozilla.org. Thanks!
Note: I am on the FIrefox Mobile development team.
MeeGo for Handsets is actually based heavily on Maemo. From our point of view at least, it's an incremental change rather than a complete replacement. Firefox for MeeGo will be an evolution of Firefox for Maemo. Of course, it helps that the bulk of Firefox code is already platform- and toolkit-agnostic - for example, we already have Qt builds for Maemo 5.