Firefox 43+ requires that extensions be digitally signed by Mozilla. This article has more information: Add-on signing in Firefox. Most publishers who continue updating their add-ons should be taking care of that soon, but it sounds as though Real wants you to switch to their newer add-ons.

As a temporary workaround, you can turn off the signing requirement but there's currently no way to only make an exception for specific extensions so it's a global change.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste xpin and pause while the list is filtered

(3) Double-click the xpinstall.signatures.required preference to switch it from true to false -- note that this is for Firefox 43-45 only

I hav not gotten the Real Downloader to work on my systen for years. I just use the download managers that are available for Firefox.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.