The "Firefox OS 2.5 Developer Preview" app for Android? I haven't tried it yet, so this is just a guess.

If you can get to Settings => Apps then you should be able to uninstall there.

If the problem is that you don't have access to Android settings while the FirefoxOS Preview app is running, you might need to power off and restart your device.

Does that let you remove it?

By the way, I noticed people discussing problems with various devices in the comments on the following blog post, in case you want to report your particular device:

https://hacks.mozilla.org/2015/11/firefox-os-2-5-developer-preview-an-experimental-android-app/

I also saw these steps for switching off the Firefox OS launcher or uninstalling:

Hi Andrew, you need to go to the Android settings. There are two ways to access them: 1) On the home screen you should see two Settings icons (the one for Android is likely darker in color), 2) After you swipe down from the top of the screen to open the Firefox OS utility tray, try swiping down again to reveal the Android one. Within that tray should be an icon to access the Android settings.
From there, if you just want to switch back to your other home screen, find “Home screen” settings. That will let you choose a different launcher. If you would like to fully delete it, then find “Apps” in the Settings (usually under a General category). You should be able to find “Firefox OS developer preview” under the Apps listing. Click on it and choose uninstall.