I've found recently that when I open a not-too-large* spreadsheet in Google Docs, within about 5 minutes of leaving it in a background tab, Firefox starts hogging 100% processor time. Closing the main Google Docs tabs and the spreadsheet tab makes it go back to normal. Is this a common problem with Firefox, or is it a problem with my extensions (eg: Firebug)?
*10-15 sheets, each of which has only a couple of dozen of rows max. No graphs or anything complicated.