I'm annoyed that neither Safari nor FireFox seem to be comfortable enough to use for long on my PowerBook.
Safari seems zippy and easy on my CPU, but crashes at inopportune moments with lots of tabs open. And since the W is next to the Q and there's no quit-confirmation in Safari, I've got yet another way to lose large groups of open tabs. And then there's all the stuff I turned off and deleted to prevent beachballs.
FireFox, on the other hand, seems relatively stable and has many shiny features. Stable, that is, except when the odd Flash movie appears and seizes the whole thing up and I'm forced to euthanize it. That, and, it seems to "leak" CPU usage--no matter how many windows and tabs I've closed, after a few hours usage, FireFox eventually grows to consume all available CPU until I quit and restart.
Eventually, one or another annoyance with whichever of these two browsers I'm using drives me to start using the other. I've got about a 14-20 day cycle going on between them.