Comment 74 for bug 1711337

@Chituc
I don't know if this is a helpful idea, or not, but here goes.
The problems are:
1 knowledge/expertise re compiling and linking specific to Firefox (sadly I can't help there - no experience)
2 Long compile times to iterate - you've asked if anyone has arm64 that could help.

Re 2, would using arm64 in the cloud help?
https://www.scaleway.com/ appears to offer an ARM64 server with 8 armv8 cores, 8GB memory, 200GB ssd on a pay-as-you-go basis (12euro a month).
They have ubuntu images, but I'd expect it would require uploading your armhf build environment & some configuring..
Not sure how much RAM you actually need, but they appear to have larger RAM configurations, with more cores, at higher cost of course.
Looking at their faq/servers page, it looks like the default ubuntu image is Ubuntu 14.04 LTS, but they also offer Ubuntu LTS 16.04 (Xenial) & Ubuntu 17.04 (Zesti)

Performance isn't guaranteed though, as this is shared infrastructure, which is probably really designed for web services..

If this looks like it would make the difference in iteration times, I'd be happy to contribute.