A few days ago, my tab key stopped working in Firefox. I use tab all the time to move through form fields, so that perplexed me a bit. I had a hunch, though, that one of my extensions might have been causing the mischief. Sure enough, after checking the Firefox forums at MozillaZine, I discovered that Paste and Go was the culprit.
After I disabled that guy, things went back to normal — I guess that one of the extension’s recent updates broke something. So, if your tab key has been acting up too, that could be worth a try. (For those who may be curious, Paste and Go adds an item to the URL bar’s context menu which allows you to paste from a URL from the clipboard and load it in one step.)