With its publication of firefox-quantum, mozilla abandoned the XUL/XPCOM technology it had previously used. Many addons had been created and over time updated for that old technology. However, those addons were not entirely lost. They have been archived here, https://github.com/JustOff/ca-archive. "The Classic Addon Archive contains 93,598 versions of 19,450 Firefox add-ons created by 14,274 developers over the past 15 years using XUL/XPCOM technology."
Be warned: Although Waterfox continues to publish upgraded versions of Waterfox-Classic based on that old technology [as well as its just 'plain' Waterfox based on firefox-quantum], its Waterfox-classic comes with the warning that there are many unpatched security risks. I have no reason to doubt that condition is also true for using 'old firefox' or 'old seamonkey. Perhaps less true of palemoon.
That noted, you can make use of that archive in Palemoon, Waterfox-classic, old-firefox and old-seamonkey.
To have access to that archive
(1) Download the latest version of XPI file from the Github web page of Classic Add-ons Archive https://github.com/JustOff/ca-archive/releases.
(2) Launch Palemoon (or Watefox-classic or firefox or seamonkey) browser.
(3) Drag-n-drop the downloaded XPI file on the Palemoon browser window. Alternatively, you can also press Ctrl+O and select the XPI file.
(4) The familiar add-on installation window will appear. Click on the Install Now button to proceed with the installation.
(5) After the installation is finished, it places an icon in the toolbar, clicking upon which you get to browse and install all the classic add-ons as if you are using the Mozilla Add-ons website.
For illustrations, see https://www.trishtech.com/2018/10/use-o ... s-archive/ from which I shamelessly flinched the above instructions.
After installation you can customize your tool bar to display the Classic Addon button:
- ClassicAddonButton.png (24.96 KiB) Viewed 626 times
Clicking that button display the following, providing access to the Archive:
- Classic Archive GUI.png (73.52 KiB) Viewed 626 times
Whether any of the addons --and which of often multiple versions of each addon-- will work with your web-browser can only be determined by your exploration.