I have a similar problem on the Firefox extension (5.0.101, Nov. 2). I can provide more details on my situation. I have 2 proxies setup to download PDFs in the extension, and on this new version clicking on the proxy menu no longer applies the proxy URL to the URL (it's not responsive). My proxy setups are still present in the options.
This happened today right after Zotero updated itself to 6.0.16, but it's likely happened when Firefox updated the extension.
This happened today right after Zotero updated itself to 6.0.16, but it's likely happened when Firefox updated the extension.
This is a known issue: https://forums.zotero.org/discussion/comment/421289/#Comment_421289