[bleepblorp]

Try severely reducing the value of mousewheel.default.delta_multiplier_y in your Firefox about:config.

[isoprophlex]

Thanks! Never realised there could be a per-program setting to fiddle with!

[bleepblorp]

Most programs don't have internal settings for this, but Firefox does things its own way--and tends to break regularly because of it.

Also try reducing mousewheel.min_line_scroll_amount if changing the other setting doesn't help. Try a value of 16.

[jdalgetty]

Is there a number you suggest?

[bleepblorp]

Firefox' scroll behavior seems to be non-deterministic on Linux, so there doesn't appear to be a universal answer.

The default is 100, which seems to work fine most of the time, but for the OP's issue, reducing it by a factor of ten might be a good starting point.