Have you checked whether Firefox is the default browser since you installed Alert Thingy? I just installed Alert Thingy on a Mac. Though I had thought that Firefox was my default browser, I found that the links from Alert Thingy were going to Safari. I then looked at Firefox preferences, and found that Firefox was no longer the default. Perhaps I just hadn't realized that Safari had reverted to being the default recently. But in any case, once I re-set Firefox as the default, the links in Alert Thingy started going to Firefox.
