Step 1: Make sure your browser accepts cookies
On Windows, go to Utilities -> Options
On your Mac, go to Firefox -> Preferences
Select the Privacy & Security panel and go to the History section.
In the drop-down menu next to the storage rules, select "Use custom settings" for history
Make sure that the "Accept cookies" box is checked and "Accept third-party cookies" is set to "Always"
Step 2: Make sure that tracking protection is disabled or only enabled in private windows
On Windows, go to Utilities -> Options
On your Mac, go to Firefox -> Preferences
Select the Privacy and Security panel and go to the Tracking Protection section.
Make sure the option is set to Never or Only in Private Windows
Additionally in the latest versions (Firefox 63):
In the Blocking content section, uncheck the All detected trackers box
Step 3: Make sure you are not using incompatible extensions
Go to Tools -> Extras -> Extensions
In the list that appears, click Disable if you have any of the following extensions: AdBlock Plus, Avast Online Security, Ghostery, etc. (open list).