How can I achieve this with Fireworks? Or what is the exact name of this technique, so I can Google it?
Graphic Design Stack Exchange is a question and answer site for Graphic Design professionals, students, and enthusiasts. It only takes a minute to sign up.Sign up to join this community
I know this is really old, but this technique is really useful, make a black square and text layer on top white; group them; go to live filters "other > convert to alpha", go to "adjust color > color fill" and choose your color in this case white, after that you can change the opacity as you want. The good thing about this, text is still editable just use the text tool or the alternate selection and double click the text ;).
You mention Fireworks but you tagged this Photoshop so this is my answer for Photoshop:
For editable text.
Fill Opacity 0and change
Place your text object above the block. Set blend bode of the text object to Erase. Flatten Text+Block.
Drawback: you have to flatten Text+Block. Both objects will not be editable (unless you save editable copies before flattening).
Convert the text to vector path (Right click >
Convert to Path). Then select text paths and do Path panel >
Join paths to join all letter paths into one. Now select both text path and block path then do Path panel >
Drawback: Both objects lose (some of) their original properties. Text is still not editable.