If I understand correct, different people use different ways of entering characters with diacritical marks, some using a Compose Key of some kind. It may be of use when you say what you use as Compose Key.
I use Shift-RightCtrl and have no problems entering those characters here in this post, which I am typing using FF.
I use right win. Now I managed to make work it in Firefox for standard combinations, but my file ~/.XCompose is totally ignored in FireFox and GTK apps, while working well in LibreOffice and KEdit. I tryed to add the custom combinations to /usr/share/X11/locale/en_US.UTF-8/Compose but this way also does not work. In Firefox and GTK any custom combinatios are still ignored. The input method is xim as reflected by the GTK_IM_MODULE=“xim” variable in the output of the “export” command.
Thanks for the additional information. I hope it will trigger others into helping you.
Because I never used any of the files you mention and I also do not seem to have a RightWin key I am afriad I am off no use for you.
I had the same problem for a long time: Firefox and other GTK+ applications would ignore the ~/.XCompose file and accept only the standard Compose Sequences, not my custom ones. Now, I was already accustomed to cursing and switching to a terminal window to type and c&p the stuff back to Firefox, but today I found a solution that fixes the problem, hopefully once and forever.
Type the command gtk-query-immodules-2.0 and look whether xim is among the supported input methods. If not, then you are in the same situation as I was 20 minutes ago. Fire up Yast and install the package gtk2-immodule-xim (plus its 32bit heritage version, plus the gtk3 versions just to be sure, alltogether four). Restart Firefox (X server restart is not needed) and enjoy.
It appears that xim input method for GTK+ is not installed by default in openSUSE (I have 12.3). Whether there is any reasoning behind this I do not know, but in the 25 minutes since I installed it nothing disastrous happened.