If that doesn't work, another possibility would be to check if you have the latest Firefox update.
I use Firefox on Windows 7, and I don't have any problems accessing it even with my ad blocker(s) on.I have. The ad blocker switch off didn't work, I also deleted everything for all time in the cache and cookies. It's a glitch since mpb's site update. It's not a problem as I can access it with Edge or Chrome, I like to browse in case I have to answer the call of GAS
Which version of Firefox? Works fine on 64.0.2 on my Windows 10 machine but I notice there is a version 65 out now which I haven't installed yet.
Works fine now after going in to settings and fiddling about, I don't know what I did??V65 Firefox on my ancient Toshiba and loads fine.