personally I don't chase shadows, there is little value in ever uninstalling (overkill) on issues such as this. It is more likely it is a back-end issue of the webrep rather than an avast installation or webrep 'avast online security'
Does it happen on all sites that you open or just avast as currently on mine it is greyed out on the avast forum, but working fine on other sites. Am I going to do anything about it, no, it is an effort of diminishing returns.
Just leave it, until you have this happen all of the time, in which case it may well be another firefox add-on interaction and that isn't going to be easy to find.