Serious help needed from the internet!

Hi Internet (photography bloggers particularly),

I really need your help. About half the time I take a photograph, photoshop it, then post it to the web, the detail and colors are totally washed out.  It’s as if I didn’t photoshop it at all.

I already tried: assigning the sRGB color profile. I already tried save for web on high quality settings. What Am I Missing?!

Oh, I will add that the color change is most prominent when I use photomatix to make an HDR photo.  The HDR qualities are completely lost when I open the photograph in firefox.  I use a mac, and I’m using the trial version of photoshop (I know, don’t judge). I shoot with a Canon Rebel xsi.

Can someone please tell me what I’m missing? I’d be so very grateful. Until then I’m too embarrassed to put up any of my pictures because let’s face it without photoshop they’re pretty crappy.

Ever yours,

T