I think that is a badge just for your information and doesn't indicate that it is incompatible with current releases of Firefox. Usually when an extension specifies that compatibility starts with a later versions, the installation button is hidden.

If you do find a problem, scroll down to the Versions section of the Add-ons page where you can check release notes and find a link to earlier releases of the extension.

Version 2017.4.19.1 Released April 19, 2017 1.7 MiB Works with Firefox for Android 48.0 - *, Firefox 42.0 and later

There can always be issues with an extension if you use the unstable Firefox 55 Nightly builds. If you want to use such a build then this is at your own risk. See also the mozillaZine Builds forum. http://forums.mozillazine.org/viewforum.php?f=23