Welcome to the forum.
I had the same problem. This is what I was advised by the company hosting my site. This is something for your IT guys to fix.
"I would therefore conclude that the solution to this problem (on Linux systems running Apache) is to add the AddDefaultCharset utf-8 directive to either the Apache config or the site .htaccess file. The advantage of the latter is that it only affects individual sites. The default Apache character set is taken from the locale file on Linux and defaults to iso-8859-1. It is the conflict between the Apache header with iso-8859-1 and the page character set of utf-8 that obviously causes Firefox a problem."
In a forum post Chrissy_Tissy added
My machine is Windows, but this fix still worked - some notes about making the fix visible:
1. Do the fix itself (httpd.conf: AddDefaultCharset utf-8).
2. Restart the box to apply the fix.
3. Once the box is restarted, clear your cache in FireFox to make sure you don't continue to see the cached file.
Once all this is done you will see the output content as expected.
See www.grainge.org for RoboHelp and Authoring tips
Thanks for your solution, it’s really working but we changed Defaultchareset in our project setup file not in httpd.conf.
Now we are facing another issue, the popup is not having TOC.
If you have any solution then please help us. We are really thankful to you