This has been addressed in the FAQ.
However, this is a browser issue. In the short term, you can always try using an alternate browser. To fix the problem, empty out your browser's history/cache and then go into your add-ons and delete them. Most aren't compatible with MyBB (at least with FireFox). After you do that, restart your browser. If you still have problems, restart your computer. If problems still persist, your issue runs deeper than your browser.