I am a big user of SLIMTIMER. Not a minute is spent at work without it running, and more and more I am using it for personal tasks as well.
I run the Timer in the Firefox sidebar. However, I have countless times used by mistake the cmd-B (or ctrl-B for non-Mac users) combination, which opens the bookmarks in the sidebar. There goes my timer: I have to reopen it and restart my current task. This is even more annoying as I don't use Firefox bookmarks but del.icio.us instead.
The good news is that I found the Keyconfig extension for Firefox (and Thunderbird) which allows you to easily edit Firefox keyboard shortcuts. Install, go to Tools -> Keyconfig, and get rid of the cmd-B binding (or assign it to something more useful). Note that I downloaded Keyconfig from here.