I’ve two ideas on improving the ability bar, that would allow one to use their abilities easier.
Instead of grouping all abilities once the bar is filled, I suggest that the movement abilities be grouped into one category and moved towards the right hand side of the bar. This would allow other abilities that are used more often to remain ungrouped for one to just click, rather than having to go into its category and then clicking the required ability.
Add hotkeys. Not much that has to be said here, but I believe hotkeys on the browser platform would improve gameplay significantly. Even more so coupled with the first suggestion.
Optional: Increase the number of abilities support in the bar. Perhaps by making the boxes smaller to allow fitting 1 or 2 more buttons.