Apparently the first public alpha of FireFox 2 is out... The two features I would like to see in Firefox (which, from the documentation I've seen, are not in the new version, and probably aren't even under consideration) are:
a) Integrating the search bar with the general URL bar. "Go to URL" should be treated as simply one behavior, with "Google Search" or whatever else being other behaviors. There's really no reason to separate those from each other, esp not when there are several plugins that allow you to select text and apply one of your Mycroft behaviors to it. It would be nice if I could highlight an URL, right-click, hit the Search Web submenu, and select Goto.
b) "Tear-off" tabs. It should be possible to "grab" a tab, and move it around among the tabs in a window, pull it out to make a new window, or move it over to a different existing window.
ETA: Upon further inspection, dragging tabs to move them (within the tabbar for a given window) is a standard feature, at least as of v1.5. *facepalm*
Also, it'd be nice if somebody would fix "Things They Left Out" (which lets you do stuff like alter the behaviors of "alt-scrollwheel", "ctrl-scrollwheel", and so on) and add support through the GUI for a few of the other nifty, poorly-documented features. (e.g. Treatment of newlines when pasting into the URL or Search bars. There's a variable you can set that makes it so the newlines get removed, or turned into spaces; this allows you to copy-paste a multi-line URL without swapping back and forth, or to paste a phyical address and google-search it. The default behavior is to just cut off anything after the first newline.)