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/

This is already set to false.

Did you have memory problems (less than 400 MB free memory) that you had to disable this feature?

What does the Firefox Task Manager report?

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-ram

Task Manager just shows what is running in Firefox, nothing special about the memory used. I have 4 GB installed on this laptop.

Another symptom is when I play a video, it will start okay but freeze some minutes later. But the audio is continuous per normal. If I stop the video and restart it, it resumes normally.