Get same performance and error messages CWL352 reported. Neither have nor use or have a process called 'better bibtex' so there is no option to disable it. Using Zotero 5.0.55 (standalone) on macOS High Sierra. Created bib file with Bibdesk 1.6.16.
So this is just the same as the other thread. You previously did have Better BibTeX installed, and you still have an old BBT translator. Reset your translators from the Advanced → Files and Folders pane of the Zotero preferences.
I can assure you I have no sneaky technology dropping BBT into unsuspecting Zotero installations. Someone did at some point install BBT into your Zotero or Firefox in the Zotero 4 era.
Hope this helps.
Thanks.
I still have never heard of 'Better BibTex', but whatever, resetting the translators worked very well.
Thanks much.