Ctrl+1/2/3/ etc... are accelerators defined to Hotlinks (quick url web access) that you can setting in: Preferences (F2) -> Browsing -> Hotlinks
Also you can modify Ctrl+Tab accelerator to show a menu with all opened tabs (similar to View -> Tabs, or right click context menu in tab or tab bar, submenu Tabs)
Go to menu: Edit -> Configuration -> Accelerators and add this code lines at the end of file:
%ifplugin tabs
CTRL VK_TAB = rebarmenu(T&abs)
%endif
K-Meleon in Spanish