Old ad-blocking methods apply on Jolla:

Get a hosts-file. Check that it does not contain anything fishy.

Enable dev mode. Set SSH server on and type the password.

In terminal type:
devel-su (the password was set in the SSH server configuration of DEV mode)

Backup and replace these files:
/etc/hosts
/system/etc/hosts
/opt/alien/system/etc/hosts

First is what Sailfish uses, the latter ones for Android.

Before replacing files it may be wise to add the original hosts file lines to the top of the ad servers blocking host file.

EDIT: Added /opt/alien/system/etc/hosts path. Seemed to help to get rid of the ad banner with Foreca app. Plain /system/etc/hosts helped with the Firefox. Hint for the alien dalvik path from the google apps installation instructions.