Thanks, I appreciate it. The 400's built in browser was unusable and I never got around to trying Firefox on it. It didn't occur to me to try another OS distro. I just used Raspian (what the box came with) and I'll presumably keep doing so. But the 500's faster cpu, more ram, and SSD, should all help.
A reddit post I saw suggested Firefox was slowed down due to a ton of small disc writes to the SD card. Maybe it's better with the SSD.