"A web page is slowing down your browser"

[DVB]

This annoying notice, on a bright yellow strip, has been appearing sporadically for the last few weeks, usually when I open several tabs on Facebook. It seems random: on half a dozen tabs it might appear on two or three of them. I'm given the options of "Stop it" or "Wait". I hit "Stop it" then refresh the tab, and it's fine.
Is it Facebook that is causing this (though it occasionally happens on other tabs), or Firefox? And is there a simple solution?

On another point, I see that this forum is about to close. It's offered invaluable assistance to Firefox and Thunderbird users for years; is there any way it could be saved? -- perhaps to be owned and run by the moderators?

[DanRaisch]

A) It's the web page(s) causing the alert, not the browser itself. As the banner says, there is a script on the page that is causing some sort of lag/delay.

B) Although the notice from the forum administrator did say the Forums would be closing, that was some time ago and no further action or information has come since. Some discussion has been held about moving but no concrete steps have been taken as yet.

[gorhill]

An extension could be causing this. Some extensions inject what is called content scripts into pages, so the warning could be because of an extension's injected content script, not necessarily because of the page itself.

[DVB]

It's only on some Facebook pages, but not others. I click on Stop, then refresh the page, and it's fine. It's been happening for a few weeks. I suspect it's a side-effect of a recent upgrade to Windows 10, Firefox or Facebook -- most likely Firefox -- or some combination of them.

My Firefox extensions are: AdblockPlus, Disconnect, DownThemAll, FB Purity, Facebook Container and Video DownloadHelper; plus two plugins I didn't request so presumably are supplied automatically by Firefox: OpenH264 Video Codec and Widevine Content Decryption Module; plus British English Dictionary; no additional themes.

[LIMPET235]

DVB,
Have you tried YouTube lately?
The same thing happens there as well, no matter which version of Firefox you run.
Tried with v20 through to v79....They all see the same "warning message."
Don't worry 'bout it.

[DVB]

Don't worry 'bout it.

Compared with Covid-19, global warming and the possibility that Trump might be re-elected, it's a triviality. But it's irritating and a waste of time, albeit minor.

[xanthon]

I was also getting this on FB. I attributed it to FBPurity. It hasn't happened for 2 months at least, probably longer.