I've checked Firefox 43 and the first barrier is the extension signing case. To turn if off you have to open
about:config and toggle entry
xpinstall.signatures.required from
true to
false. Then it'll be possible to install unsigned extensions. If disabling compatibility check won't work you'll probably have to modify the metadata of the extension.
Example for
Change Referer Button from Pale Moon Add-ons website. You have to download the .xpi file manually, open with some kind of .zip archiver, extract
install.rdf file and open it with a text editor. Then just add target application info for Firefox under Pale Moon's entry:
Code: Select all
// some properties at the top
<!-- Pale Moon -->
<em:targetApplication>
<Description>
<em:id>{8de7fcbb-c55c-4fbe-bfc5-fc555c87dbc4}</em:id>
<em:minVersion>25.0</em:minVersion>
<em:maxVersion>30.*</em:maxVersion>
</Description>
</em:targetApplication>
<!-- Firefox -->
<em:targetApplication>
<Description>
<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
<em:minVersion>25.0</em:minVersion>
<em:maxVersion>43.*</em:maxVersion>
</Description>
</em:targetApplication>
</Description>
</RDF>
Save changes in the file and put it back into opened .xpi archive (confirm overwriting). You should be able to install this example extension also in the newest Firefox.