Unless someone decides to re-write the program to work with Quantum, This link is as close as you can get; https://www.askvg.com/firefox-tip-restore-old-classic-options-aboutpreferences-page/

I will revert to "insecure" Firefox 55 and I know how to prevent updates (I use linux). Too bad the developers think we should all be clones. I disable my car alarms and rewire power windows so I can use them without a key. The end user should be able to decide what they want.

See also:

• https://github.com/Aris-t2/CustomCSSforFx/issues/2

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