Firefox 53 switched off the neon detection paths due to importing a new version of skia; see
https://bugzilla.mozilla.org/show_bug.cgi?id=1289569
and related issues.
The current version of skia, however, still supports non-neon builds at compile time... this is a very dirty patch to achieve this against Firefox 55. Unfortunately it's not complete, as I was building it on one of Google Cloud's attached SSDs, and forgot to generate the patch file of the final state. You'll also need to revert most of:
https://bugzilla.mozilla.org/show_bug.cgi?id=1345267
To make it work. However, the release build in this repo includes that revert and works fine for me... if you trust random apks...
I'm only likely to run a build for this against Firefox 55, because 56 is dropping support for ICS (Android 4.0), and hence for my Sony Tablet P.
If you want a Tegra 2 compatible browser for JB (4.1) or later, see also: https://github.com/Decatf/chromium_src