Firefox: Inconsistent Volume Levels On Youtube

Hello

I've had a problem with Youtube volume levels in Firefox, for many versions, now. I have just updated to 57.0.3 (64 bit). The sound level on Youtube videos is lower than with other browsers ( Chrome. for example). I've gone in to the volume mixer and raised the slider there, to equalize them.. But, a little later, the problem returns.
Is there some fix that I've missed while googling?

Josey Wales said:

Are you using any Legacy Extensions? Do you have Flash installed and Updated? How many Processes do you have it set for in Options? I use 4. I do not have A problem with sound.

Thanks for the quick reply!
Added an additional Youtube flash selector & made sure 4 processes were selected... so far so good! Will see if it "holds"!