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FYI: The site keeps crashing the tab in Firefox 115.7.0esr.

You should report it to Mozilla, nothing a site does should ever crash a browser.

(Also, it could be an extension)

With the possible exception of "consumes a lot of memory, leading to the OoM killer being invoked".

No, even excessive memory usage should be prevented by the browser. If any page can crash a tab/browser it's a browser bug.