From: http://lifehacker.com/#!244497/remove-misspelled-words-from-your-firefox-dictionary

1. Open your application data folder. On XP or Vista, go to your Start menu and hit Run (or just press Windows-R) and paste in %APPDATA%\Mozilla\Firefox\Profiles; on your Mac,
navigate to ~/Library/Application Support/Firefox/Profiles.

2. Find your profile folder, which by default should look something like xxxxxxxx.default.

3. Inside your profile folder, find the file called persdict.dat and open it up in your text editor
of choice.

4 Now just find the misspelling, delete it, and you’re done.

I imagine it’s similar for Opera. Good luck!

@ninwa Thanks! So then this begs the question: Why does Opera think ‘loosing’ is correctly spelled?

After a little more research, in Opera, you can right click on any word (inside a text box) and remove it from Opera’s spell-check dictionary.

@ninwa O.O. Wow. That was awesome! Welcome to the collective, please stick around.

@MyNewtBoobs – Thank you! I’m glad I could help. :) This site has so much charm and personality, I think it’s fantastic.

As far as “loosing”, it’s an acceptable variant for “let loose,” such as “loosing the hounds.”

@crisw So that’s why it thinks it’s ok. Well, now it doesn’t – I mix it up with ‘losing’ every. single. time. So it’ll remind me now, and I’ll just ignore it for the few times I use it like in your example. Thanks!