Most of what's below is probably FreeBSD-specific, but this issue will spin off into something more general …

Disconnect 3.15.3… ⚠Not compatible with multiprocess.
$ pkg info waterfox | grep Version
Version : 56.2.0.13
$ date ; uname -v
Tue 22 May 2018 18:24:45 BST
FreeBSD 12.0-CURRENT #2 r333587: Mon May 14 01:59:02 BST 2018 root@momh167-gjp4-hpelitebook8570p-freebsd:/usr/obj/usr/src/amd64.amd64/sys/GENERIC
https://addons.mozilla.org/addon/disconnect/versions/ presents 5.18.21 as compatible:
Works with Firefox for Android 48.0 and later, Firefox 48.0 and later
– and https://addons.mozilla.org/en-GB/firefox/search/?q=disconnect&platform=FreeBSD&appver=56.2.0 does not find the Disconnect extension. @jbeich this is reminiscent of _add-on services: Tier-3 platform BSD equivalence to Tier-1 Linux · Issue #4610 · mozilla/addons-frontend_ but that's a _front-end_ issue so I'm taking care to not link on this occasion:
https://github.com/mozilla/addons-frontend/issues/4610
https://addons.mozilla.org/addon/disconnect/versions/?page=1#version-3.15.3.1-signed.1-signed presents the installed version as incompatible and:
Works with Firefox 1.5 - 31.*
Maybe negligible. I think of Mozilla bug 1394225 - multiprocess (e10s) automatically enabled at launch after installing extensions that are not compatible with multiprocess, comment 2 in particular.
There's one here:

– so we might expect a user to be unsurprised by _later_ discovery that what was installed is incompatible. However there's a complication …
FreeBSD is not a platform officially supported by Waterfox. Can you reproduce the issue with a supported operating system? Please verify, otherwise a fix might be extremely hard to find, let alone done.
- install Add-on Compatibility Reporter 2.2.4
AMO says "This add-on has been removed by its author."
- observe the list of found extensions, with no invitation to 'See all nn results' at its foot
Try replacing %OS% with Linux in extensions.getAddons.search.url. Le Même en Mieux is the only missing addon. Maybe _parseAddons() returns aTotalResults == 0 on FreeBSD.
https://github.com/MrAlex94/Waterfox/blob/436898372f8589d4a241e924084119358c58bf45/toolkit/mozapps/extensions/internal/AddonRepository.jsm#L1226
https://github.com/MrAlex94/Waterfox/blob/436898372f8589d4a241e924084119358c58bf45/toolkit/mozapps/extensions/content/extensions.js#L2690
Disconnect 3.15.3 at the head of the list
about:addons before mozilla/gecko-dev@0a51e1ac6452 showed results inline using a different API, see resource://gre/modules/addons/AddonRepository.jsm.
Results don't appear to vary between platforms
but about:addons probably filters out non-compatible based on
<all_compatible_os><os>Darwin</os><os>WINNT</os><os>Linux</os></all_compatible_os>
Thanks,
Add-on Compatibility Reporter 2.2.4
https://github.com/mozilla/addon-compatibility-reporter/releases/tag/2.2.4 (via the _Notes_ section of https://redd.it/8c77ii – apologies for the omission).
…
extensions.getAddons.search.url…
Ah! extensions.update.url was suitably modified here weeks ago …
… I forgot about extensions.getAddons.search.url. Making the string Linux-specific causes Disconnect to not appear amongst results, which at first sight seems strange to me, but …
… I'm not in a rush to follow-up. A head-spinning day with extensions (identifying a possible bug in Extension Conflict Troubleshooter, then using the extension to identify issues involving three others, etc.) so I probably need to sit on this issue 578 for a few days. Regain head space.
… probably FreeBSD-specific, but this issue will spin off into something more general …
Yellow alerts
… a complication …
Pfft … the stuff that I wanted to spin off from there is no longer in my main profile. (I might have been trigger-happy with housekeeping a few months ago. Dug, found some screenshots from August 2017, not enough to jog my memory. Hopefully it'll come back when I get head space!)
Doh
Oh, comments crossed paths, sorry yours didn't appear until after I posted mine. I'll probably take a look later in the week. Thanks again.
@grahamperrin, can you reproduce using www/firefox-esr (while it tracks 52.*)? Even after mozilla/gecko-dev@aeee1208082b AMO can limit to specific platforms (i.e., all_compatible_os -> platform). Try asking Disconnect folks to publish the addon with platform=all.
Fixed by freebsd/freebsd-ports@83794bc14760.
@jbeich
Assuming this fixes the issue, why don't you create a pull request for this? I think everyone using Waterfox on a platform not officially supported could profit here.
_Off-topic:_ Same goes for issue #416, there you have brought up a fix, but that fix never made it into the repository. Please, if you have a fix for any issue at hand, create a pull request for it. Otherwise the issue is never going to be fixed.
@Peacock365, the fix here is a barely tested ugly hack which needs more work: conditionals to not break addon compatibility on Windows/macOS, submitting upstream (Firefox) for review and anything else I've missed due to unfamilarity with addon machinery.
Ah, I see. In this case I assume it is better to wait until Mozilla has approved the fix (better safe than sorry).
And I didn't know that #416 is already fixed, nobody ever mentioned it in the issue itself.
Off-topic II: Some love for issue #373 would be nice as well, as you can see here Mozilla didn't bother to fix the bug at this point: https://bugzilla.mozilla.org/show_bug.cgi?id=1415921#c0 If you could fix it this would be really nice.
OK,
can you reproduce using www/firefox-esr (while it tracks 52.*)?
Maybe next week, although with time constraints etc. I can't promise.
@jbeich in the meantime we have the positive test result https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226919#c17 for (marginally outdated) www/waterfox 56.2.0.13_1. Big thanks.
… Try asking Disconnect folks to publish the addon with
platform=all.
A smart idea, however I'll defer any such request until after another issue (OT) is addressed.
www/firefox-esr
@jbeich I haven't found time (sorry), if I do then I'll follow up in a FreeBSD area.
Pfft … the stuff that I wanted to spin off … is no longer in my main profile. (I might have been trigger-happy with housekeeping a few months ago. Dug, found some screenshots from August 2017, not enough to jog my memory. Hopefully it'll come back when I get head space!)
I have not yet found what was lost, what I needed for the (non-FreeBSD-specific) spin-off issue.
It's unlikely to resurface in the near future so we can close this issue 578. Thanks folks.
Most helpful comment
Fixed by freebsd/freebsd-ports@83794bc14760.