Now is this being done with two tabs in one window? If you could, try loading each Amazon account on separate windows of Firefox.

I know I could open a new window but that's not as convenient. I like having the two tabs side-by-side and switching between the two. It hasn't been an issue for years and suddenly it starts doing this. I would like to be signed in to my different accounts in two tabs within the same window as it has been for a long time.

Firefox Sync copies selected data in encrypted form to the Mozilla cloud to share between different installations of Firefox. It doesn't share any data between websites.

This could be due to a change by Amazon, or possibly you were using an Add-on to isolate the tabs from one another that has become disabled for some reason (e.g., MultiFox).

Mozilla is testing a new feature called Containers that you can use to isolate instances of a site side-by-side, assuming the site allows it. More info: https://testpilot.firefox.com/experiments/containers

Reading up a bit on Amazon, do you have this kind of account:

North America Unified Account
In North America, we have simplified managing your business across the United States and Canada marketplaces. The North America Unified Account enables you to create and manage product offers in both the U.S. and Canada. You control what you sell and where you sell from a single seller account interface.

When you sign up to sell on either Amazon.com or Amazon.ca, your seller account is automatically enabled to sell in both marketplaces. You can switch between marketplaces using the marketplace switcher in Seller Central. This allows you to list products and manage orders in each marketplace.

From: https://services.amazon.com/global-selling/register-overview.html (North American marketplaces section)