lets puts it this way the ++ctrl+PageUp++ shortcut is a quite common shortcut to switch to next tab along with ++ctrl+PageDown++ which does the opposite with go to previous tab
You can map the key's escape sequence directly as in this question. Vifm handles input through curses library which doesn't support every possible key.
vifm :tabn :tabp why isn't it implemented in vifm unlike nvim?
:tabnext and :tabprevious are implemented in Vifm.
also <s-pageup> with ++shift+pageup++ doesn't seem to work either.
Don't know if that's even supported by any terminal.