Photoshop has been the standard image editing program for designers for many years. It is definetly a powerful program with tons of features. I use it for print work only, as Fireworks seems to be geared primarily for web and screen work, and is less bloated with features I won't use. The latest version of Photoshop, though, is very nice to work with. It's fast and intuitive and definetly worth learning
The color control in Photoshop is far more advanced and exact than any other program I've tried. Of course, Adobe wrote the book on this stuff. I wish that vector drawing tools within Photoshop worked more like in Illustrator does (Fireworks maintains vector shapes from Freehand files, and you can draw just like in Freehand within Fireworks too). Other than that, it's the best for what it's for, even though it's not perfect.
TipsManage Symbols in Libraries
Your project may have reusable elements, colors and other elements used in your designs. Create a library with these objects so that if/when changes are needed, you can easily update the original symbol to update all of the designs that use that element.
Pull in fonts from TypeKit and Google Fonts for use in your designs to ensure consistency with the final web or app project.