Those 2 programs are made by the same company (since Avast bought AVG), and that might answer your first question.
Can you tell 4 things:
1. What virus is it?
2. Does it happen if you open Firefox or if Firefox opens its home page (easy to check by setting the home page to about:blank)
3. Does it happen also if you open another browser?
4. Does it still happen if you reset Firefox to defaults (google that if necessary)?
Was using Avast! for years; a couple weeks ago began getting a warning about the program capturing and putting into virus chest the same virus over and over. Uninstalled, reinstalled, same issue. After days of doing this, uninstalled and then went to AVG. Same exact issue, same exact warning and always one right after the other (in both programs), same verbiage. How can I kill whatever is causing this annoying issue? Happens whenever I open Firefox?