I trust you mean: "I haven't been able to get ...."
Since you're running Mac OS X 10.6.7, what version of Firefox are you using? I'm running 10.7.5 with Firefox ESR 45.9 (the last version usable by Lion). You might find that that version could resolve the issue.
Maybe not, but it can't hurt to give it a shot. And if not, there's likely something faulty with Protonmail, which they're not likely to fix for a no-longer-supported browser.