In UI3 version 27 released Saturday (June 2nd) I added an "HTML5" player which works in all the modern browsers. This is the first version where Firefox has an H.264 Player dropdown list in UI Settings, allowing you to choose between "HTML5" and "JavaScript".
A few days earlier (Thursday), in UI3 version 25, I added a "Native" player which I renamed to "NaCl" in version 27. This player is only available in Chrome and ChromeOS and is due to stop working some time this year due to Google removing support for the framework this player was built on. You won't ever see this one in the dropdown list in Firefox.
That said, Firefox has an issue with the new HTML5 player. You'll probably notice it stutter/pause a lot, especially if you are streaming at 1080p or 4K. There is a complex reason for this but suffice it to say Firefox doesn't handle low-delay streaming well. Mozilla has known about the problem for years but chose not to fix it. There is a workaround you can try. The stutter gets worse when it has been a long time since the last keyframe, so you can reduce the stutter by reducing the keyframe interval in
Blue Iris's web server streaming profiles. The lower the better. However high values result in much better image quality and bandwidth usage (that is why the default is 300!), so this is really not a great workaround. Best to just not use Firefox for UI3 as long as this is an issue, or use the JavaScript player which is fine as long as your CPU is fast enough.