Mozilla recently issued a major upgrade to the Firefox browser (version 36.0). In this version they have (evidently) changed/increased some security settings. These changes have prevented our users from being able to use Firefox to login to our web site administration system.
After weeks of this issue biting me in the backside, and hours of Googling/researching this, I FINALLY figured out how to get around the recent issues with Firefox and the admin.
There's one setting in Firefox, which once you change it, will allow Forefox to work with the admin system again.
Here's how to adjust the setting:
1) open a new tab/window in firefox. In the actual address bar, type "about:config"
2) it will give you a warning, click the button which bypasses it
3) near the top of the page, there is a search bar
4) type "security" into the search bar. It will automatically do the search, and display the settings containing the word "security"
5) look for the setting "security.mixed_content.block_active_content" (its about 19 settings down in the list)
6) the value is likely currently set to "true" (I think thats the default).
7) Double click that setting which will change the value to "false" (it will also change the setting to all bold, which evidently indicates it has been changed from the default setting).
Then go to your admin login page, and see if it will let you in. Ive tested this on another desktop system here at my home office, so I have a high degree of confidence this will work.
Oh, and in my case, it worked, without closing the browser and re-opening it.