A few months back we settled on a UA String for Firefox on iOS. The universe still seems (mostly) intact so it should probably be documented.
Similar to Chrome for iOS, we're using the default Safari Mobile UA string with an additional
FxiOS/<FxiOSVersion> token. For example,
Mozilla/5.0 (iPhone; CPU iPhone OS 8_3 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) FxiOS/1.0 Mobile/12F69 Safari/600.1.4
Here's a handy FAQ:
Q. As a web developer, what should I do now that I know this? A. Probably nothing.