For the last couple of months, I've had an incredibly annoying problem in Firefox, when entering text into text boxes: the cursor would disappear, the text box would appear to lock up, and I'd have to click inside the text box to resume typing. Often, too, I'd hit the Backspace key and this would jump me back to a previous page, causing me to lose what I'd been writing. I did several searches on this problem and finally located the answer here. I had set several bookmarks in my beloved Speed Dial extension to automatically update every hour or every four hours, etc. For my Gmail, I had it ping my inbox every few minutes. It was these dial-outs that robbed the focus from the text boxes: as I typed, Speed Dial was checking a web site and updating the thumbanil, and Firefox -- unable to serve two masters -- left me bereft.
Not wanting to disable Speed Dial, I instead set the bookmark updates to Never, and this seems to be working.