No, the TH is not pronounced like "F". Sometimes people will say it that way because they are not being careful, or because this happens to be a common and acceptable pronunciation in a given community or according to a common idiom or dialect, or because they know they are not in a formal setting. Pronounce it as you have been formally taught.
It is pronounced, thecca the th is like Th is there not like f.
Ðð is pronounced as the th in the English word though.Þþ is pronounced as the th in the English word think.
The letter: F (pronounced 'ef') lower case: f
thawm ثوم (th pronounced as in think)
thady ثدي (th pronounced as in think)
katheer mina'l'hobb (th pronounced as in think)
Θ, θ [theta, pronounced as "th" in "think"]
thalaathoon / thalaatheen (th pronounced as in think)
estheseakA αισθησιακά "th" is pronounced like in word "think" or "thought" not "that" or "then"
Byddin - the by is pronounced bu as in bum the dd is pronounced like the th in the bu - th - in
TheE moo TH is pronounced like the TH in the word "thought" or "think" not like "then" or "this" theE moo
"Even though" is pronounced "EE-ven TH-owe". ( IPA = ˈivən ðoʊ )