I just upgraded to Vimium 1.60. An important piece of functionality appears to have regressed.
Previously, if I were to press f and a link target ABC were to appear, then I could press ab<Shift-c> to open the link in a new tab. That is, fab<Shift-c> behaved in the same way as <Shift-f>abc.
This is no longer the case. Now, pressing <Shift-x> for any x while in f-mode deactivates f-mode. That is, pressing f<Shift-a> or fab<Shift-c> is the same as pressing no key at all.
I noticed this behavior on Chrome 60.0.3112.90. I then upgraded to Chrome 61.0.3163.79, which has the same behavior.
(Edited to clarify key notation.)