Is it via your provider's website?
. . . or are you using a dedicated email client on a PC (such as Windows Live Mail, Outlook or Thunderbird)?
. . . or are you accessing your emails through a phone app?
No not as advanced as that. Just an old fashioned computer with monitor and big keyboard. I use Outlook for emails - the mails come up ok - then I double click on them to read them (this usually opens them) but recently they have been a headache.
I was hoping that you wouldn't mention 'Outlook' because Microsoft branded two entirely different things with that name and it's easy to get confused!
When you want to view your emails, do you use your web browser (as you're doing now to read this post) and go to the Outlook website or do you use a completely separate program (which happens to be called 'Outlook') which is solely for your emails?
The 'Error 502' message in Firefox suggests that there's an out of date cookie (or something similar) in Firefox's cache. Clearing the cache and removing cookies ought to fix the problem.