The Go>Up shortcut should be Alt+UpArrow instead of Ctrl+U.

The motive is that Alt+UpArrow is a natural extension of the similar scheme
of Alt+Left for Back and Alt+Right for Forward (bug 48317), and for that
reason, it's easy to remember. If you know any of those other shortcuts
already, there's a chance that you will try Alt+Up and see if it does what
you would think it does. Arrow keys are usually tied to navigation, and
they are positioned so that it resembles navigational controls. I don't
think that can be said about Ctrl+U in a similar manner. It's more of a
random key in the middle of your qwerty keyboard that could have any action
tied to it, any action with an U. And it lacks the perception of being
directional because it is neither placed with symbolizing direction in mind
nor is it marked with a symbol that is reserved for directional use.

On a cognitive level, using the Arrow keys for navigation is both easier to
"get" and remember because it takes a navigational concept and uses the
most natural visual mapping of this concept that we usually are trained to
recognize; directional arrows. If you have experienced directional arrows
before, then knowing what it symbolizes is an experiential process. The "U"
key on the other hand is reflectional because you have to remember both
that is tied to a navigation action (because it lacks all visual clues of
being tied to a directional action), and that it represents the direction
"Up" (because you have to map it to first character of the literal word "Up").

"U" also localizes more badly, you cannot expect users with another first
language than English and whose language word for "Up" doesn't have "U" to
get that last word matching at all.