I looked to see what other browsers I could test with. Underlying engines are getting fewer these days, which presumably is why Firefox is worried about being left behind by, say, Chromium based browsers.
For completeness though, this seems fine on Chrome, Edge, Firefox 122.0.1, and Pale Moon which is related to Firefox long ago from a fork of its Gecko engine. I also searched Mozilla Bugzilla days back, finding nothing to indicate there is a general JavaScript bug, yet we have one. I reported it to Mozilla…
They’d probably prefer if it was a web site they could get to over the Internet, but that’s hard to arrange securely, so we’ll find out what happens with http://localhost:8200/ngax/index.html#/edit/12 which is where my installation happened to be when I was playing with the button and JavaScript issue.
Although behavior is reported above as a little unpredictable, I don’t do JavaScript or web work, so test that I tried was to start with no filters. Adding one in a working browser seems to give a default-to-tweak Exclude expression on the dropdown, * in the typing area, making three-button-menu Edit as text of -*, and being able to reverse back to original dropdown-plus-typing area form using Edit as list.
Firefox 123.0 fails in both directions, and it’s not anything we changed. Even years-old Duplicati breaks.
EDIT:
On Linux Mint 21.2, it works on 122.0 and fails on 123.0 after I let OS’ updater update. This was before rebooting, so it was the same running Duplicati. Only obvious difference was that it’s now Firefox 123.0.