I updated to Zotero 5.0.81 today and found that I couldn't access the Zot2Bib menu in Zotero; I clicked on the Zot2Bib button and the drop-down menu wouldn't appear.
Then I looked at the Zotero changelog and noticed that there is a note in the changes in 5.0.78 (December 1, 2019) that says: "Zotero 5.0.78 includes many internal changes, and some plugins have not yet been updated for compatibility. The most common plugins (ZotFile, Better BibTeX, Zutilo) have been updated."
So I downgraded to Zotero 5.0.77 (I had to replace my Zotero data folder with a backup in order to downgrade—always back up before upgrading!) and found that the menu still works, so I am sticking with Zotero 5.0.77 for now.
I am not using one of the latest supported versions of macOS, so it would be great if somebody else could test this with a recent version of macOS and confirm that the issue can be replicated.