Yes, This is done using the file associations within FlashFXP.
Add a new file association with
*.html as the file mask, then change the Open With value to the internal editor, notepad++, or whatever you'd like to use.
Using an external editor such as
notepad++ is highly recommended for the best experience.
The order of the file associations used by FlashFXP are as followed:
1. Compare against the internal file associations for a match, if found open with the defined program, otherwise continue to #2
2. Compare against Windows file associations for a match, if found open with the defined program, otherwise continue to #3
3. Open the file using the default file viewer/editor defined in the Preferences dialog under the View and Edit File section.