Can you clarify the issue that you’re having with Firefox under Linux?
I use Firefox to access Ipfire from both windows and Linux without issues.
A couple of points:
The Ipfire Web GUI doesn’t use cookies so I’m not sure why deleting (other pages) cookies or site data would fix the issue temporarily for you.
The CGI scripts run on the Ipfire web server. Pretty much anything that you see in your web browser from Ipfire comes from a CGI so I’m not sure what you mean by ‘always carried out’.
Are there any error messages that you’re seeing either from Firefox or on Ipfire ‘/var/log/httpd/error_log’.
I would check and see if you have some browser add-on installed under Firefox that’s causing issues for you.