In relation specifically to the ones you mention, diffs and abs are not widely used. Abs to most people means abdominal muscles, or workouts that concentrate on those muscles.

If you are being extremely casual (and a bit silly) you can use 'obvs' to mean 'obviously' - but it's very slangy and not recommended beyond anyone you know very well.

If you were talking, for example, about school lessons, you could shorten lesson names to three or four letters - Geo (geography), Sci (science), Chem (Chemistry) etc. If someone said 'bio' in most contexts this would mean a biography of someone, rather than biology. In specific contexts such as school lessons, biology could be shortened in this way but it would be unusual.

Obviously sci-fi is a common shortening, now a word in its own right, short for science fiction.

Sci-fi is pronounced /saifai/. That is to say, the "i" sounds are pronounced as in "science", not as the "i" in fiction. Both syllables are stressed, the second one has the primary stress in most accents. The "i" sound of fiction is not phonologically possible in a stressed open syllable in English, so it certainly could not be used in the abbreviation "fi". (It's also not really possible in an unstressed open syllable in most people's accents).

Thanks, Julia, and how is sci-fi read? /saifai/ /saifi/ /sifi/?