Update 22 November 2023:
Apparently, YouTube have been slowing down Firefox users and especially users with adblockers enabled.

I thought it was a weird coincidence that both my Firefox and Mercury (a fork) browser experienced massive slowdown on YouTube. It wasn't just loading the YouTube pages, both of the browsers can't even load YouTube videos as fast as they should.

Both browsers were able to load YouTube videos at normal speeds again after disabling this plugin. I've tried disabling and enabling multiple times to be sure. Some Firefox users reported that they did not experience any of the slowdowns regardless of the news. It could be the usual experimental rollout that YouTube always do and perhaps my browsers' fingerprints changed the moment I disabled this addon.

As of now, YouTube is fast again even with this addon enabled.

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It has been a great companion to my Malwarebytes Premium. However, recent updates have killed its efficiency and performance, making Firefox extremely slow.

I've noticed huge slowdowns every time I open Firefox. Not only that, I lost the ability to watch videos above 1080p with this extension enabled as it is simply killing Firefox behind the scenes for no apparent reason.

I only found out this extension to be the cause of it by using Firefox's troubleshooting mode. As soon as I enabled this extension, Firefox became very sluggish and slow.

I'll write them an email about this but for now, avoid it at all costs. If you already have Malwarebytes Premium installed on your PC, I don't see a reason why anyone would need this companion extension.