Well, someone (won't name him) did post here but removed his reply. 
Anyway, does that mean that web-shield works for you with Firefox..?
My post/s had nothing specifically to do with the Web Shield (which is enabled), but about the OPs comments and choice of Subject.
No Internet Browser works with Avast mobile security Android other than crome
The fact is the Firefox browser works just fine for me with Avast mobile security for Android installed.
I wasn't clear if this was about AMS (which I believed) or the Web Shield. However I have just browsed amazon .co .uk with the web shield enabled and subsequently checked my statistics and the Websites scanned has increased.
So all would appear fine on my AMS Android, with the web shield enabled on Firefox for android APP.