A couple of years back I managed to set it up somehow that I could write ae to get ä, AE to get Ä, oe to get ö etc. Without pressing the compose key.
Similar to swedish-postfix input method in Emacs, but it worked for all windows, browsers, terminals etc and I could toggle it.
But I have no idea how I did this. On Debian, how do I get started with installing input methods like this and activating/deactivating them, actually using them?
Outside of Emacs, which has this built in and doesn't need an external input method for it, I typically use dwm, rxvt, Firefox and Inkscape.