Unless you are doing something different, you are using Firefox to access your mail provider's website. The data remains on that site until you remove it. Note: Junk mail and deleted folders are cleaned out now and then.

What about any bookmarks or saved logins you set up in Firefox? Firefox Sync should help you recover those when you set up a new installation of Firefox, but it's not completely reliable. This article suggests how to make a backup of your Firefox data:

Back up and restore information in Firefox profiles

Actually, you probably should back up everything on that disk that you wouldn't want to lose.

Thanks for your help

Glad to help. Safe Surfing.