Maybe related to this feature ?

Firefox will detect if your computer’s memory is running low, which is defined as lower than 400MB, and suspend unused tabs that you haven’t used or looked at in a while.
The new tab unload feature is controlled on the about:config page with a preference that you can set to false to disable this feature.
Keep in mind that this makes you vulnerable to low memory issues.

• browser.tabs.unloadOnLowMemory = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

• http://www.ghacks.net/2019/03/01/firefox-67-automatically-unload-unused-tabs-to-improve-memory/
• https://blog.mozilla.org/blog/2019/05/21/latest-firefox-release-is-faster-than-ever/

Thanks for your reply. This preference is already set to false, obviously by default. The game freezes immediately after loading a website in the new tab. Just noticed that even when i minimize FF to tray and then maximize FF again, the game freezes also.

On normal Firefox Edition (78.0.1 64-Bit) i have set the preference "browser.tabs.unloadOnLowMemory" to true, no freezing of HTML5 games could be detected. The Problem only affects latest Firefox Developer Edition (79.0b1). Just updated Firefox Dev to 79.0b2, the problem ist still there.

You can check the Firefox Task Manager (about:performance) and about:memory pages.

• https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox
• https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-or-cpu-resources

I've just installed the latest update. With 79.0b5 (64-bit) there is no freezing of HTML5-Games.