The extensions I have had installed for some time are BLUR (by Abine, tracker blocking, masked e-mails) and AdBlock Plus. There were no unwanted surprises in Preferences (nor in System Preferences LogIn items).
I deleted the quarantined file, reran AV and malware scans. Have not had any alerts.
Just for information, what currently appears in ~/Library/Safari/Extensions are Extensions.plist and Blur.safariextz. The .plist file is incomprehensible but does contain a block of stuff about Blur.
\Apple-signed_Hash of Content Blocker String_Bundle Directory Name_Added Non-Default Toolbar Items_Archive File Name_Bundle Identifier_Developer Identifier[Hidden Bars_Removed Default Toolbar ItemsWEnabledYSignature_Has Called canLoad _Blur-2.safariextension†_Blur-2.safariextz_com.abine.dntpsafariZ82M4SELZCB††......
The next blob goes on and on and with many URL sort of things that either have something to do with AdBlock or whose presence makes no sense. Since I've never examined this file before, I can't know.
I think I'll uninstall those extensions, make sure the files get trashed and reinstall. I'll report back to confirm whether the stuff in ~/Library/Safari/Extensions looks okay or not.
Update:
Appears the long blob is from the adblocker. I was also having trouble with very slow response in Hotmail which I thought might be related to this in an indirect way. Uninstalling everything associated with AdBlock Plus eliminated that issue. I installed a new one, uBlock Origin, which seems to play well with everyone. Checking Extensions.plist, once again there is a long list of stuff; that must all come from the ad blockers.
p.s. The audio Verification option doesn't work. The captchas do because I can't read some of them.