I don't think it's a bug, but intentional. Thunderbird is cross platform so won't necessarily recognise platform-specific features.

If you compose using HTML, look in the menu for Insert and there you'll find an option to insert commonly used characters.

Otherwise look for the abcTajpu addon which has a compose function.

No, it's defiantly some sort of bug because same version of firefox can do éèòú etc. on win8.1. But if i were to type the exact same on my laptop it would look like this. ´´e``e``o´´u for those 4 characters.

You probably have more than one keyboard installed in your Windows 8.1 and one of them is causing problems. Press Alt+Left Shift once to change to the next installed keyboard and see if that helps.

If it does, you might want to remove the erroneous keyboard by clicking on the taskbar icon with the language code indicating your keyboard and going to the settings.

I do not. Neither do i have any problems on my win8.1 machine, as mentioned it worked just fine. On my laptop running win10 there's only one language installed and in that language there is only one keyboard language. And the problem is only affecting mozilla products.

Edit: mozilla products and ms office, not sure what that's about.