ctrl + a doesn't work in browsers

Solution 1:

Had the same problem in 12.10 and it was driving me mad. The problem seemed to be caused by using the Emacs key binding theme as described in the links given by others, except that according to GConf I was still using the default.

I realized that this is now set in dconf Editor irrespective of the settings in GConf.

To change this open dconf Editor and navigate to:
org > gnome > desktop > interface >gtk-key-theme.
If the setting there is Emacs change it to Default.

Or on a terminal:

gsettings reset org.gnome.desktop.interface gtk-key-theme

Problem solved. For me at least.

Solution 2:

I had this same issue happen in both Ubuntu 12.10 and Mint 13. The way I fixed it was by going in through ccsm and changing my themes back to defaults.

The Gtk+keybinding theme EMACs was the issue, when I returned it to default it fixed the issue and all other themes were working fine.

Solution 3:

Not sure what's wrong with your keyboard shortcuts, but this might help for Firefox; enter about:config in the address bar and then 'select' && the two options that can be useful are:

browser.urlbar.clickSelectsAll

browser.urlbar.doubleClickSelectsAll

(Just set them to the opposite of default :))