In Firefox 100 on Kubuntu 20.04, I see that Adblock Plus 3.12 doesn't and won't yet update to 3.13. I don't see why. Perhaps later.
I have the same issue with Firefox 100, Win7 64-bit. I see the available update message but doesn't/can't update. Weird.
Just now, I noticed this info: "Note: Due to third-party store review policies, it may take a while until the update is made available to everyone." https://blog.adblockplus.org/releases/adblock-plus-313-for-chrome-firefox-microsoft-edge-and-opera
Thanks, JRViejo. Same here, in Firefox 100.0 on both my Kubuntu 20.04 systems. (Also Firefox addon 'I don't care about cookies' got an update, to version 3.4.0.)