Hmm, I'm not sure what that error indicates. Is that an error in the web page when you try to download it, or something that occurs after you give Firefox permission to install? Where does it appear in the process?

As a reality check, if you go to a page on the Mozilla Add-ons site, right-click the Add to Firefox button, then Save Link As, can Firefox successfully download the file? As a point of comparison, if I do that for

https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/

I get a file named "ublock_origin-1.67.0.xpi" with a size of 4,152KB in Windows File Explorer. Do you get that or does Windows block/remove it?

Thanx for getting into it. The Error comes from the About:addons page. The message comes up directly after pushing the "Install"-button of any of the additional pages. Pls. see the attached screen shot. The error message is displayed at the upper/right corner.

My best guess is, that noting was downloaded and the error message is because of an empty result.

Reality Check: If I try to get https://addons.mozilla.org/firefox/downloads/file/2527685/two_litle_birds-2.0.xpi. When I try to get it in FF, I receive the same error message immediately. When using MS Edge to get the same link, it delivers a 122KB file named "Nicht bestätigt 100451.crdownloadas" as a part of the whole which needs further DL which never happens.

Mine's disabled so I haven't tested, but does your new install have "Smart App Control" enabled?

https://support.microsoft.com/en-us/windows/app-browser-control-in-the-windows-security-app-8f68fb65-ebb4-3cfb-4bd7-ef0f376f3dc3#bkmk_smart-app-control