I was going through all my 'old' electronics, and came across a bunch of Frsky receivers, and a LOT of spare antennas for them - Would I be able to use these antennas on my Crossfire receivers (Instead of the Immortal T)? Since I switched to Crossfire I don't like the thought of having wasted all that money on antennas, and it would be great if I could still use them.

No, unless your FrSky receivers were 900MHz R9 receivers. The R-XSR and XM+ receivers are 2.4GHz while TBS Crossfire is 900MHz. The antennas are tuned to the corresponding frequency of the receiver so you cannot use a 2.4GHz antenna on a 900MHz receiver, and vice versa.

Oh well, there was hope LOL, thanks for clearing this up for me

Some of the 2.4 antennas were pretty long, you could try stripping out the shielding to leave 1/4 wave length unshielded. For 900 mhz guess that would be roughly 8.3cm, not sure how good the reception will be though. Also note that some of the Frsky antennas had smaller ipex4? connectors which may not fit on your crossfire rx.