I’m posting the following suggestions for people who, like me, have been frustrated trying to download videos on Firefox from YouTube, other video sources and – theoretically – even porn sites.
Here’s what worked for me (I’m running FF x64, v. 89.0, on Windows Pro 10):
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Find and install two FF add-ons,Video Download Helper and ANT Video Downloader. (If you get an annoying message saying the add-ons can’t be installed because of yadda, yadda, save the file to your hard drive, then open it with FF and give approval for its installation.)
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Activate the Bookmarks toolbar (f you haven’t already) and move the icons for both add-ons to it, next to each other, using the Customize command. This makes them easy to see and use.
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When you’re om a page with a video that you want to download, first click on Video Download Helper. Sometimes it will only offer a low-resolution choice; in that case, going to the “settings” icon in the lower right corner of the video will often allow the choice of a higher-rez file.
After choosing a resolution, click on the arrow next to it pointing to the right and select “download” (NOT “quick download”). That will allow you to designate a download folder. (For subsequent videos, just click on “download” – the add-on will automatically download the file to the last folder you selected.)
- If Video Download Helper shows nothing available for download, try ANT Video Downloader. It will often show more download options. Be patient – wait at least 10 seconds for available options to load. Choose one which says segments ARE available for download. After making a selection, a dialogue box will open (eventually), asking you for the download destination.
In my experience, Video Download Helper will show downloadable vids about 80 percent of the time. ANT will show other valid choices about 10 percent of the time. For about 10 percent of videos, neither will work – and occasionally the download will fail anyway. (In those cases, it’s the site, not the add-ons, that’s causing the problem.)
Nonetheless, a success rate of close to 90 percent is pretty damn good, if I do say so myself. 
Unfortunately, there’s no convenient add-on to download photos more quickly than using the traditional right-click “save as” command. In that case, I stick with the Pale Moon variant of FF and use the Get ‘Em All add-on (a fork of the now-obsolete Down Them All). Sometimes, right clicking on an image page in Pale Moon and choosing Get ‘Em All will give the options of downloading all of them at once, instead of one at a time. Fast, reliable and free – what’s not to like? But where’s the FF equivalent?
Hopefully, other users will add more suggestions to this thread. But the procedures described above work well for me … most of the time.