In the Gmail client, I often go to the drop down menu (3 vertical dots) and choose "Filter messages like these" to sort future emails into one of the many "labels" I use. I tried to do that earlier today, and the dialogue box that normally opens when I chose "Filter..." did not appear. I tried it with several emails - still no dialogue box. I then opened Google Chrome (see attached image), went to the same email in my Gmail client, and the feature worked. I suspect that the issue with using the feature in Firefox is that I upgraded to macOS 10.15 Catalina last night. I am using Firefox 70.0 for my browser as I prefer many of its features over what I have in Chrome or Safari. Please let me know if you are aware of this issue, or whether I am an outlier, having a one-off problem.