Obviously you need to identify what AVAST is flagging as an “Unreputable toolbar” by which I think that means the toolbar, if it exists, is of unknown repute rather than disreputable.
But whilst you wait for one of the AVAST gurus here to help sift through the log for the culprit you could be checking for anything obviously unwanted that you may have inadvertently installed with another program.
Have you looked at your Firefox toolbars in “View” and “Addons” > Extensions to see if anything has been added to Firefox you don’t want?
You could try using an uninstaller which has a specific option for removing browser toolbars. That will show you what is there if Firefox is not listing anything untoward and remove it should you so wish.
If there is an unwanted toolbar one of those toolbar uninstaller programs may be essential along with some additional manual cleaning too.
