In addition to Meho's response I can add a couple of tips.
There are only a couple of programs today that can do automatic ligature substitution, InDesign and one that I can't remember, but it's not commonly known or used. Hopefully Scribus will soon join the group. I love ligatures and have used them since before InDesign came out.
In the old days I did my writing in MS Word, where I recorded a macro to do the ligature substitution (tip: search and replace for the ffl and ffi before the others). Today I would do the same in OpenOfice Writer. Now, the problem with this is that once you have run the macro your spell checker will flag all the words with ligatures as misspelled. In other words, you have to do it as the very last thing before importing the text into Scribus. And if you make any changes to a word with ligatures in Scribus you'll have to add the ligature manually. Fortunately Scribus does have a handy ligature option in the Insert menu.
Another point is that you need to check the font to be sure it has a complete ligature set. Today I would use Fontmatrix, a free open source font viewer. Fontmatrix will also tell you the Unicode value for the ligatures, essential when doing the search and replace. Some fonts have no ligatures at all, yet others have not only the f-ligatures (fi, fl, ffi, ffl, fj), but also old style ligatures like ct and st where the c and s are joined to the t with a curve. The latter are useful for setting something designed to have very old effect.