I have a document in epub format open in Firefox, but can't find any option to search for a specific word. It only has option to search within the page currently viewed, not the whole document. Is there such possibility, or alternative ways to search the epub file?
You can try a different ereader such as Coolreader. Additionally, epub documents are zip files usually. If all else fails, you might be able to unzip it and then do a textual search on the individual html source files.
I came across the same problem. I found the answer here and I downloaded "Coolreader". In fact, it didn't work. Maybe there are bags in this program. Anyway, I found other free program (Calibre) which works very well now for searching in the entire book.
I am posting this answer for others who come this post by googling and may spend time by downloading the program which is mentioned here in the previous answer. (this is the the article that I found this program in. There are other programs there but some of them didn't work properly and some of them I didn't try since I found already what I wanted.