I get this a lot also. FF crashes. Kills the filter I was working on - it goes to 0kb and can't be loaded.

(I can locate the ffxml file using the locate file on one of my similarly named surviving filters, though.)
Seriously, this seems to happen at least once every few hours of FF use. I've become compulsive about saving backups. If anyone could tell me why it's doing this, it would be much appreciated! (some memory thing, maybe?)
Occasionally, it crashes so bad that it 'forgets' the filter structure. That is, all my downloaded filters disappear from the filter browser. (!) If I download a filter from the FF website, it's able to right itself. However, if there is no internet connection, then there are NO downloaded filters. (my filters are preseved.) It also deletes all of my 'favorites' when this happens.