Add-ons • Re: need help understanding how to install from the Classic Add-ons Archive

What it means is that the version you found isn't strictly compatible with Pale Moon due to the way older extensions did compatibility checking - sometimes it maybe a restartless extension (if it has a no-restart label on or near the description). You can still download the extension and manually install it from the addons page (Press Ctrl-shift-A, or open about:addons in a new tab or Tools menu->Addons. Click the hamburger menu on the addons page and do 'install addon from file').

For each extension page on CAA, you'll see a link that says 'View all x (number of) versions', which will take you to a list of all previous versions of that addon. The one that's directly compatible with Pale Moon will show an install button next to it rather than a download button.

In the end it doesn't matter, you can save an extension locally and then manually install like I said, or you can use an older version of the extension (which may lack features added later, or have bugs that were fixed later). See whatever works best.