Im wondering if anyone ever figured this out. Basically i tested this at length on two different Mac and 3 different version of the OS. Currently my new mac arrived and i once again created the same copy/paste issue within Firefox.
MAC: 10.15.6 Catalina 32GB RAM, 8 TB SSD .. a good machine
Firefox version is now 79.0 (64bit)
Avast version is now 14.5 with the lasted definitions.
I installed Avast and just the 3 fundamental core sheilds.. File, Email, and Web.
If the Web Sheild is on, open firefox... copy and paste doesnt work in Firefox, but does in Chrome and Safari.
If i turn off just the Web Shield, close firefox, and open firefox.. copy and paste works fine, but now my web surfing is unprotected.
I have tried Avast Web Shield with the "scan secure connections off as well to see if it had anything to do with that, it doesnt appear to change anything related to copy and paste.
I also noted that some people are suggesting killing the process pboard, which of course is the pasteboard. This doesn't fix the issue, it breaks Avast hooks into the the pasteboard/ clipboard.. literally breaking the app temporarily. Thus i dont see this as a solution any more than shutting off the web sheild
I have also posted this on the Firefox forums and i am awaiting responses. I have searched alot over the last month to find others with issues. Some of the other posts doent seem to have solutions, just people arguing. Would love to know if this is fixed, or someone found a solution.