Persistant pop-up ads in Steam and Firefox even with Adblock

Hi all

Like a lot of other users, Ive been experiencing pop-up ads in both firefox (despite have Adblocker installed) and Steam. This has also been occurring on a few android devices, however im convinced that that is non-related.

I have formatted both C drive and Steam drive, installed and ran AdwCleaner, MalwareBytes, McAfee Security Centre and Microsoft Malicious Software Removal Tool (not at the same time). The issue still persists.

Any help would be appreciated.
Thanks