It looks like the FF 58 requirement was added because AMO has retired the use of SHA-1. This algorithm was previously for signature verification purposes. Basically, AMO signs an XPI so that Firefox (or other tools) can verify that the file has not been modified since AMO signed it. Add-ons are now signed with PKCS7 SHA256 and COSE ES256, which is only available in Firefox 58 and above.
EDIT: When I first came to this thread I also thought @stig’s suggestion would work, but I’m pretty sure that this signing change means that even if you remove the minimum version field from the manifest, users on older browsers will encounter an error when attempting to install that version. That may be worse overall for those users than keeping the minimum version flag and preventing updates from reaching them.
I empathize with your desire to keep your add-on working for as many users as possible. While I would strongly encourage these users to update to a newer versions of Firefox due to a plethora of security issues with old web browsers, it’s ultimately their choice to stick with 56.
If you want to go the extra mile for these users, I see two options:
- Make an unsigned, non-updating version of the add-on available on your (project’s) website.
- Self host and distribute a version that can automatically update itself. If you go this route, you’ll need to make sure that you properly configure and host an update manifest for your add-on.