This extension makes hot keys work with Russian and Ukrainian "йцукен" keyboard layouts in Linux and *BSD (needed only for GTK2 builds). Don't try it with other languages/layouts, the result will be unpredictable. The problem with hot keys is well known to all Russian Mozilla users, it was reported over 6 years ago (bug #69230) but is fixed only in Firefox 3 beta 2. The extension is a hack, but it solves the problem for previous versions.
Since 1.4 there is also experimental support for Hebrew and Russian/Ukrainian Dvorak layouts. Hebrew should work out-of-the-box (except keys with "Q" and "W"). To activate Dvorak mode go to about:config and specify integer "extensions.rhkbf.layout" preference (create it if it does not exist). The values are: "0" for Cyrillic and Hebrew "йцукен" (default) and "2" for Cyrillic Dvorak.