ALL DATA AND ATTACHMENTS UNREACHABLE
Zotero was saving data and attachments within a directory in ~/.mozilla/firefox/9is9uuc7.default
Suddenly, on opening the app (Ubuntu 20.04, Firefox 83) it failed to find the directory, made an empty new one in ~/Zotero/, containing just empty templates, but no info.
The directory is still there, and I can access it through gnome with no apparent problem. Moving the subdirectories of 9is9uuc7.default/ to Zotero/ makes no difference. When I go to preferences and try to change the file directory, it fails to find the firefox, and really the file explorer interface only identifies a small number of directories, either hidden or not hidden. Am I missing something?
Suddenly, on opening the app (Ubuntu 20.04, Firefox 83) it failed to find the directory, made an empty new one in ~/Zotero/, containing just empty templates, but no info.
The directory is still there, and I can access it through gnome with no apparent problem. Moving the subdirectories of 9is9uuc7.default/ to Zotero/ makes no difference. When I go to preferences and try to change the file directory, it fails to find the firefox, and really the file explorer interface only identifies a small number of directories, either hidden or not hidden. Am I missing something?
Zotero 5, which was released in 2017, automatically migrates the data directory (the 'zotero' folder within '9is9uuc7.default') to the new default location, ~/Zotero. If you hadn't upgraded to Zotero 5 in years, or something on your system prevented Zotero from moving the directory, it would continue using the directory within the Firefox profile.
It sounds like what happened now is that you installed a third-party Zotero package that's sandboxed, such that it no longer has access to the data directory within the Firefox profile — and that's also why it only shows a small number of directories when you browse for files.
We can only provide support for the official Zotero tarball, but to fix this all you need to do is move the contents of ~/.mozilla/firefox/9is9uuc7.default/zotero — zotero.sqlite, 'storage', and other files — to ~/Zotero, such that ~/Zotero/zotero.sqlite exists. Zotero will then use that database.
Before doing this, you should delete everything else that you previously copied to ~/Zotero (or move them back to the Firefox profile), since it sounds like you tried to copy your entire Firefox profile there. Those files have nothing to do with Zotero and shouldn't be there.
And let me tell you something: thank you for a very quick answer during a crisis, I appreciate that a lot. It was a moment of sheer panic!
Again: When your third-party, sandboxed package of Zotero lost access to the data directory in the Firefox profile, it reverted to the default data directory location of ~/Zotero, which was empty.