I may be wrong (I like to believe I am not lol), but I don't think you RAM is the main issue... C55D I believe (which quick research) is an AMD E1 CPU which is only 1GHZ & dual-core. You are bottle necking the brain power of that laptop.
Browser now use more processing power. Not just Firefox (Chrome is worse). I recommend looking into a invest of a processor that is 2GHZ or quad-core.
Upgrading your ram gives the PC more "hands" to hold things, metaphorically, to hold more applications running at the same time.
CPU is more about actually processing things, or "thinking" and "doing", what you have told it to do by clicking and typing, (essentially almost everything on the browser), especially videos playbacks.
I have a feeling you need a better processor, which in turn is a newer laptop, since you can't upgrade a processor on a laptop. (For the sake on sanity and reasonable investments, don't try either lol).
@CA3LE why you gotta make this a whole Edison / Tesla war man...
But when i said Edison killed Tesla, I meant literally, killed him... I don't remember, but I think it was a hired hit...
I don't have my facts straight.... hmmm