In Firefox Developer Edition I can install the add-on from the add-on store.
When I download the xpi file and try to install from file it also fails on Firefox browser. When I unzip the xpi file, then I can install it in Firefox Developer Edition.
It looks like there’s a signature problem in the signed file. I don’t know why that happened. My suggestion is that you upload a new version (with a higher version number, doesn’t need to have different code) and see if that fixes the problem.
Users are complaining that they are unable to install due to this error
“Installation aborted because the add-on appears to be corrupt.”
Which runs fine on my development machine FF 71 / Mint
However for new installs I can reproduce the install error on the latest stable FF 72.0.1 / win 10
the js console does not provide any additional information
The extension is mature (last touched in 2016) and it will be a challenge to find the original files and build script although I should be able to hack the XPI if its the matter of dropping a file and resubmitting at a higher version for resigning.
I was able to install it with no problems on Mac OS. If you can reproduce the error, I suggest looking at the Browser Console for any installation errors.