How to fix issue with Firefox being slow with proxy

I have been struggling with a slow browser when I visited particular sites while using a proxy from a particular provider - could not figure out why and then I discovered the following fix:

go to the firefox menu ≡ > options > privacy & security panel and under permissions check the setting to prevent accessibility services from accessing your browser.

Made a big difference.

Just thought I would share in case anyone else is having the same problem.