Crop is a destructive action, yes, but you can do it more precisely or undo it.
You can use the crop tool, which comes with nice adjustment handles, then press the enter key to crop. That tool is the lower right of the four Select tools at the top of the toolbar.
When I want to be precise about the dimensions, I'll make a rectangle the size I want, position it, then hide my bitmap (and anything else). To crop, I'll just press Ctrl-Alt-T (it's the keyboard shortcut for Modify > Canvas > Trim Canvas), which trims the document to the rectangle. Then, I'll delete my rectangle.
As for undoing the crop, Ctrl-Z (on Windows). Unlike Photoshop, you can go back quite a number of steps with Ctrl-Z.
I never use the marquee for cropping; it's a selection tool. You have many better options, none of which use the marquee tool.
If you are talking about a single image file, try either masking the image (rather than destructive cropping) or change the preferences for Fireworks so that cropping doesn't delete any data.
If the image is part of a design. Select the image and choose Edit > Crop selected bitmap (destructive) or mask the image (non-destructive).
Another alternative is to choose the Export Area tool, and export out your crop as a separate bitmap file.
Masks, yes, masks are great! If you just want to hide parts of your image from view, create a mask. Masks can be other bitmaps or vector objects. Create and position your mask object over your original bitmap. Select both then Modify > Mask > Group as Mask. You can easily undo the masking. White areas in a mask reveal the underlying object. Black or transparent areas in the mask cause your original image to be transparent for those pixels. Grey or semi-transparent produce a proportionally transparent result.
There's also the Paste Inside option, where you can paste your bitmap into a vector shape. The nice thing there is that you can move your bitmap around. (I'm pretty sure, anyway. I don't use this feature.)
Anyway. You have lots of options to remove or hide pixels. The approach you take depends on your needs and preferences.
If you have an image to share and a goal to achieve, we might be able to advise you better.
Yes the masking function is perfect.
The Paste Into function does just what I need.
Just like in Photoshop, you can lock or unlock the companion mask.
But you can also scale the vector info that represents the mask.
Hey, that's great!
Yeah, you can do quite a few cool things with Fireworks! I think different programs (or program classes) use terminology somewhat differently. Or perhaps they just enhance the basic operation in different ways as the programs are developed.