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1/ I cannot see any mess on that page anywhere. And for that matter, nor can I see any ticks, or boxes, or whatever Avast-related there. Tested with IE and Chrome. Don’t have FF installed.
2/ No such settings as far as I can tell. Then again, the normal behaviour is shown on the screenshot in this post. You get the ticks right to links on certain pages (such as search engines) and only when you hover that with your mouse cursor, you get your “intrusive” box.
3/ The only action I get from the browser extension on the Stack Overflow page is blocking a bunch of social/tracking junk./ So, all in all, if what you are reporting is that you get some green boxes without any interaction with the page, then it’s most likely Firefox-specific cosmetic issue.
4/ The browser extension is entirely optional.
Finally, if you consider those “intrusive”, check things like WOT, the hover infoboxes are whole lot more “intrusive”.
If you want support to respond, that’s what support tickets are for. There’s no reply guaranteed anywhere on this forum, though Avast staff reads and responds quite often here. You need to consider, however, that this is a cosmetic browser specific issue worst case, so realistically extremely low priority for any development purposes.