The extension "abcTajpu" (a b c type-oo) offers a number of ways to enter the non-ASCII letters from various European (and some other) languages. The simplest way to type an accented letter, when you're in a text field of a web page, is to press alt-k (for example, pressing the sequence alt-k , c can be for ç). It's also possible to context/right click a text field and follow the abcTajpu menu. To configure "abcTajpu" and create your own macros, keyboard or content menu click on options/opcioj button.
So, by pressing alt-k the Abctajpu popup will appear, and if the first checkbox is ticked (⚡) then you go straight to the first text entry box (for macros) and when you enter a valid macro the popup automatically closes and the macro expansion is inserted into the page. For example if the Abctajpu options defines macro c=ĉ and the ⚡ is ticked, then typing alt-k c will insert a ĉ. Due to Firefox bug 1324255, instead of typing alt-k c, you currently have to type alt-k tab tab tab c.
The key alt-X provides a reduced set of alt-k functions, but without the need for the popup dialogue or the right-click menu. So on a text input field in a webpage it is possible to type for example c alt-x so that ĉ is entered.
For Firefox on Android, only the popup (as new tab) is available - no context menu or alt-x key binding (which are not really appropriate for mobile devices),