In Firefox, click the 'hamburger' (three lines, top right).
Click 'Add-ons'.
If it's not already selected, click 'Extensions'.
Check that your adblocker is shown there and that it's not shown as 'Disabled'. (If it is then, obviously, click to enable it).
If the adblocker is shown as enabled but it's not working, click to remove it. Then install this one:
https://addons.mozilla.org/en-Gb/firefox/addon/adblock-plus/
(If your old adblocker isn't even shown, use my link above anyway)