... Let us just revisit Tab Tally, shall we?
https://addons.mozilla.org/en-US/firefox/addon/tab-tally/versions/
All versions currently listed on AMO are, of course, of the WE format; version 1.0.0 is advertised as being compatible with Firefox 49.0+, versions 1.1.0 through to 1.3.0 advertised as being compatible with Firefox 48.0+; I was particularly interested in one of those older WE versions, because they duplicate the functionality of the XUL ("legacy") version 0.1.1, now removed from AMO (but is recoverable via the CAA extension).
All (WE) Tab Tally versions 1.0.0 to 1.3.0 install and function properly in Firefox ESR 52.9.0[1] ; "properly" is actually the "desired-by-me" way (constant visual display of the number of tabs, updated in real time when that changes...); of course, the latest version 1.4.0 also installs there, but with a slightly degraded (IMHO) functionality...
But trying to install any one of Tab Tally versions 1.0.0 to 1.3.0 in Serpent 52.9.0, you'll be met by a failure to install in the form of message:
That's because v1.0.0 to 1.3.0 are id-less WEs (one only has to inspect their manifest.json files) and Serpent 52 lacks the API to allow installation of id-less WEs (but FxESR 52 does support that API).
In version 1.4.0 of Tab Tally, the author re-introduced the "WE-id" feature inside the manifest.json file:
"applications": {
"gecko": {
"id": "@tab-tally"
}
}
and made it compatible with even older Firefox versions (min version lowered from 48.0 to 42.0) and, probably unbeknownst to him, also restored St52 compatibility; 1.4.0, as discussed previously in this thread, would install fine in St52 (but not v1.0.0-1.3.0, which do install and work fine under Firefox ESR 52.9.0... )