When using several "blocker-type" extensions with-in a same browser's session, how is URLs/payloads blocking handled?

For example, what happens when Firefox is loading a webpage while uBlock Origin AND Privacy Badger are running together? Same question if one uses uBlock Origin AND Adguard, etc.

Is one extension reading and blocking then the other doing the same once the first one is done? So in this scenario the second app would have a lot less work to do, but in thi scase which extension starts first?

But I would be surprised this is how it works. So in the other scenario, how could both filter the same page at the same time, especially if both extensions are supposed to remove something in common?


1 Answer 1


In most browsers, the current page is handled by a single thread, so the extensions will execute sequentially, meaning not in parallel.

I believe that extensions that listen to certain events will get called one after the other, so the first one will win and the others won't even know that it happened.

Browsers do not implement the preemptive concurrent execution of extensions, since the required complexity more fits an operating system than a browser and requires multiple cooperating threads.

  • Any way to know which extension will start first? Or is it completely random?
    – Ozwel
    Feb 21, 2023 at 15:04
  • 1
    That depends on the browser. Most likely it will last-one-first, or possible first-one-last, but there are no documentation for it (logical, since the implementation may change in the future).
    – harrymc
    Feb 21, 2023 at 15:05

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