I'm afraid that following the recent update to Firefox, there have been significant changes that have meant the Donation Reminder isn't compatible with this at present.
We are building a new version that will be compatible with the latest version of Firefox and when this is ready we will update this article to let people know. To receive this update, just log in to the Help Centre and follow this article and you will be emailed with any comments that are added.
In the meantime, you can continue to use the Donation Reminder on Chrome and Internet Explorer, and we have an app available on both Apple and Android devices. You can also always shop through our website as well.
I hope this helps, if you need anything else please let us know.
The new Donation Reminder for Firefox has now been released, please let us know any feedback you have as it has a new look and feel to it as well.