While Forms was a new software product, back then in its 3.0 version (or even lower), you could choose whether you'll keep the form source
- in the database or
- in that case, you could have written a query which selects data from the data dictionary and - hopefully - extract tables' names
- in file system
- file extension was .INP (not .FMB) and it was a textual file; it means that you could even create a form using text editor! Nobody probably did that, but hey - you could have done it.
.FMB is no longer textual file. Yes, you can open it it a text editor (such as Notepad++) and search for e.g. FROM
(because any table used in form's PL/SQL units or LoVs is part of a SELECT
statement which requires the FROM
keyword) and get something like this:

Yes, you'll get "duplicates" if any table is referenced more than once.
Another option is to write a program which will parse the .FMB file and extract tables' names (I can't help with that, though).