A couple of times now I’ve had to hunt down the location of the dictionary for Firefox because in the popup for a misspelled word has ‘Add to dictionary’ too close to the word I want to change to (and now I’ve inserted a misspelling into my personal dictionary).

The location of the dictionary for Firefox (under Windows) is: somewhere under Documents and Settings is the file persdict.dat.

Maybe this time I’ll remember it. I suggest this behaviour shows a usability problem.