I have the same problem, also on Lubuntu. This problem appeared somewhat recently.
I have tried updating Firefox to the latest version, and also with a freshly installed Firefox-ESR (latest version), both from Mozilla's repository. This did not solve the problem.
Also, drag and dropping files works just fine on Chrome on the same machine.
It works for me, opens the file in browser as usual.
My original install was .deb through ubuntu repositories, then from Mozilla's repository. Neither worked.
This is what I get:
Hope this can help.Code:firefox: Installed: 128.0.3~build1 Candidate: 128.0.3~build1 Version table: *** 128.0.3~build1 500 500 https://packages.mozilla.org/apt mozilla/main amd64 Packages 100 /var/lib/dpkg/status 128.0.2~build1 500 500 https://packages.mozilla.org/apt mozilla/main amd64 Packages 128.0+build2-0ubuntu0.20.04.1 500 500 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages 500 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages 128.0~build2 500 500 https://packages.mozilla.org/apt mozilla/main amd64 Packages 127.0.2~build1 500 500 https://packages.mozilla.org/apt mozilla/main amd64 Packages (plus many more previous versions)
BTW,
You can add a file to Google Drive by sending it to the associated Gmail address (if you have one), and then clicking the "add to Drive" icon on attached file in the email.
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