uBO doesn't apply cosmetic filters unless added via picker and only for the session
at least for images (from https://github.com/mullvad/mullvad-browser/issues/127 )
STR add cosmetic filters, save/apply
||www.google.com/images/branding/googlelogo/*x/googlelogo_light_color_272x92dp.png$image
||www.google.com/images/branding/googlelogo/*x/googlelogo_color_272x92dp.png$image
visit google.com .. image loads
now add the filter from the picker (doesn't matter if it adds a duplicate entry), the image is removed. open a new tab, close the current google tab, open google, image is blocked.
restart, check filters (they are there), load google .. image is not blocked
note: can't reproduce the issue on ESR102 starting PB mode, or any FF releases