Notice that the finale of season 5.1 (“Gliding Over All” ) could have been called “271”, the name of the Walt Whitman poem from which the actual title is derived. “Felina” is the elements Fe, Li, Na (as some people guessed), but that’s the easy part.

The title cards of Breaking Bad, like the periodic table, show the
atomic numbers of each element (in the green squares). So Fe=26, Li=03,
Na=11. So “Felina”, as a number is 260311. But Felina is an anagram for
Finale, so rearrange Felina (260311) to Finale (231106). Put the
half-finale and the finale numbers together, and you get 271, 231, and
106. These are three titles of poems from the Whitman book from Gale
that incriminated Walt. Ready for the answer? Drum roll, and… “Gliding
Over All, To a Pupil and to a Common Prostitute.” If you don’t think
this is a reference to Jesse and Wendy, the poem “To a pupil” references
Jesse’s idea about building a magnet, and the poem “To A Common
Prostitute” tells the prostitute to remember the speaker’s name.