I'm just an end user commenting in case it adds anything to the work on the browser. I'm not a programmer at all. Congratulations on such fast browser on our old machines.
I have a MacBookPro5.1 with an Intel Core 2 Duo, 2.53 GHz bought around 2009 with Snow Leopard. 10.6.8. I upgraded the ram to 8GB with OWC memory, and got rid of the internal superdrive to add a second hard drive with an OWC upgrade. I moved the original HDD that came with it to the old superdrive location and replaced the main drive with an OWC SSD 480 GB. Big performance boost with that SSD.
I've kept it on Snow Leopard all the way through to today because it was amazingly stable and I heard bad things from others who had upgraded their OS's. When I finally realized I was going to need a more modern OS for some things, instead of changing anything, I formatted a bootable external 180 GB OWC SSD with El Capitan, the most modern OS approved by Apple to run on this computer. So I have in effect two computers. I boot up to Snow Leopard for 95% of what I still do, including browsing. If I need the more modern OS, I just boot up from the external drive. Very elegant, and zero complications. El Capitan runs quite well.
But I still do most of my browsing in Snow Leopard. Aside from Tor, which is up-to-date but very slow, my un-updated Chrome and Firefox are sloooow and crashing now, a lot. Especially Chrome. Safari is defunct. Stainless does only a couple of things. Opera is okay, not great. Roccat became useless.
So it was with great interest that I learned about Arctic Fox. Downloaded 64 bit 10.6 and up version and fired it up. Did not import any old settings or bookmarks. WOW. What a contrast. Lightning fast and, frankly, a better browser experience in terms of speed than any I've used since 2009. Even a site like Salon, which drowns me in videos and pop-ups, was very fast to scroll and move around in. For about 20 minutes, I was in heaven.
Then everything crashed. Gone. So, I restarted the browser, restored the tabs. Soon crashed again. Now it only goes maybe five minutes between crashes. Re-booting does nothing to help. I have no add-ons at all. The constant crashing makes it barely usable. I can't have a 2000 word email crash. I know this was an early build and so buggy, but I thought I'd let you know my experience in case this was some clue that might help you to stabilize it.
When it runs, it's amazing. Totally primo. I'd kill or pay to get a stable version of it.
Just one man's story ...