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Firefox Power User Keeps 7,400+ Browser Tabs Open for 2 Years
(www.pcmag.com)
A place to discuss the news and latest developments on the open-source browser Firefox
I'm not sure if it fixes your problem, but it fixed mine.
Pin the tab.
In my case, Whatsapp web didn't get loaded when I opened firefox, so notifications didn't reach me unless I opened the tab at least once.
If the tab is pinned, however, it will load when you open firefox, I'm unsure if the tab stays loaded.