I've got used to this behavior in Internet Explorer but hadn't found any extension for Firefox that does the same so I started a new one.
Current version is 0.4.1 (install, version history, known bugs). Should work in Firefox 4+ in Windows (32bit/64bit) and Linux (32bit/64bit).
Supports:
- Changing keyboard layout when switching between tabs/windows (including sidebars)
- Configurable default keyboard layout for new tabs/windows
- Opening link in new tab/window preserves keyboard layout
- Preserving independent keyboard layout for address bar globally or on pre-tab basis
- Preserving independent keyboard layout for web search bar and page find bar
- Session restore support
- Undo close tab/window, tab drag-and-drop and tear-off support
- Panorama support
addons.mozilla.org page