Did you know that you can get a Nightly version of Firefox for Android without getting your hands dirty with Mercurial or a compiler or even have to launch Eclipse at all? Yes, you can! Here's how!
There are two ways of accomplishing this. First I'll show you the scenic route, which involves connecting your Android device to your computer:
Download the latest build of Nightly for Android from http://nightly.mozilla.org/. Just in case of doubt, it's in the Mobile row, and it should be an .apk file. If unsure about choosing the ARMv6 version or not, just choose the non-ARMv6 one. Download the Android SDK tools (if you don't have it already). We basically need the adb tool to upload the apk into your Android device, so you only need the SDK tools and not the complete Android Studio or the ADT bundle. Click on "Download for other platforms" to show the links to android-sdk_rxx... files, and download the one that corresponds to your platform. For example, in Mac it is android-sdk_r22-macosx.zip.
Now, uncompress the file and place its contents somewhere sensible. I chose ~/bin/android, but anywhere is fine as long as you can reference adb (i.e. try to avoid spaces in the path!).
In your Android device, ensure you are allowed to "Install apps from unknown sources", and also that the debugger fires up when you connect the phone to the computer (i.e. you don't want the storage to be mounted as a multimedia device). This sort of depends on the version of Android and ROM you're using so I can't give you specific advice. Also, it seems that recent versions of Android hide the Developer options menu, and you have to press the "About this phone" option repeatedly until it reveals the hidden option (!!!).
To make sure that we can continue, connect the device to the computer with the adequate USB cable, and open a terminal window:
(but replace /path/to/ with the appropriate path in your case. Example: ~/bin/android/platform-tools/adb)
If you can't see any device listed or an error like this:
error: device not found - waiting for device -
then your setup is not working. So go back and recheck the previous step.
Otherwise, we can just use adb to install the Nightly APK file in the device:
/path/to/adb install /path/to/downloaded/nightly.apk
In my case:
~/bin/android/platform-tools/adb install ~/Downloads/fennec-24.0a1.multi.android-arm.apk
After a while you'll get a confirmation message, more or less like this:
3678 KB/s (29590130 bytes in 7.856s) pkg: /data/local/tmp/fennec-24.0a1.multi.android-arm.apk
meaning that Nightly is installed in your device! You can now launch it and experiment with the latest features, without having to wait for the new version to be released into the regular or beta "channels"!
The second (and way quicker) way to get Nightly in your phone is to just download it with your phone! Using your browser, go to http://nightly.mozilla.org/ as before, and download the APK. Then you should be able to install it "using the Android UI", as Mike Cooper mentioned in the comments.
But even nicer is that you only need to do this once. After that, Nightly will check for updates itself, so no need for more cable plugging or console fiddling! You can also deactivate the "Allow to install apps from unknown sources" and "debugging when cable is connected" settings, if you want to.
Thanks to the mighty potch for the pointers to the nightly APK builds!