I personally have a Nexus 7 running Android 5.1, which I use to test Chrome and Firefox. I also have an iPad and iPhone that I use to test Safari. Quill works great in all of these platforms. Quill also uses SauceLabs to automate testing of every push. I have filed browser bugs in the past on mobile Firefox, but have been told sometimes the issue is from the keyboard driver itself, which is two layers from the browser. With the distribution of Android versions and the differences between devices, it is cost prohibitive for an open source project to test and catch all Android issues.
Of course none of this explanation helps your particular situation. What you can do to help your particular situation is:
1. File a bug report so others can reproduce, identify, and fix the issue: https://github.com/quilljs/quill/issues/new
2. Encourage SauceLabs to support Firefox on Android as a testing platform.