I bailed on Firefox recently because I grew tired of the usability regressions and bugs with the UI.
Pressing F6 no longer focuses the address bar.
The 'New Tab' button is gone from the list of optional buttons. Why?
The title bar text in often unreadable because of the transparency effect.
Worst of all is an issue with the menu bar, address bar, and bookmarks bar. On both of the desktops I use (one at home, another at work), if I minimize Firefox and then come back to it, those three bars will be in a different order than last time. No, I am not kidding.
Goodbye Firefox.
EDIT:
I just installed FF5 to see what it's like, and right away there are problems. Most of my important plugins are "unsupported". All of the bookmarks I deleted after migrating to Chrome are back for some reason. The minimize, maximize and close buttons are missing from the upper right corner. That's either a bug or another giant usability fail. I can't wait for Firefox a few versions in the future; it will probably just be a semi-transparent box with no buttons or functionality.