By default mouseover events expose a more descriptive tooltip, or an extended tooltip--when offline local help is installed and has an article pointing at the button.
And with Assistive Technology (AT) support enabled, on mouseover of a button control the descriptive tooltip is sounded by AT instead of the button label.
When keyboard navigation is used: e.g. F10, F6, TAB/ShiftTab, or cursor UP/DOWN/LEFT/RIGHT--the AT sounds *only* the abbreviated button label. Same for menu label, and context menu label which also sound just the short label.
If possible, for better accessibility support, keyboard navigation should offer the same audible/visual rendering of the more descriptive tooltip(s) provided with mouseover event of buttons.
A rework of label structure for menus, context menus and toolbar button labels is being looked at for bug 108458. Perhaps in conjunction with that, some structural change to facilitate use of tooltip/extended tooltips with keyboard navigation might be implemented.