In fact, I have to say that I am a bit confused and discouraged. It worked, but now it don't.
First I flashed the whole firmware found
somewhere on this forum. And It worked. I was back to the original firmware with a lot of languages (the C67ML one). I then installed a few apps without problem. Then I installed FDroid (and I should say that I forgot to say I made the same mistake the first time, so it was probably not Firefox fault) I rebooted and the JDRead got stuck in a bootloop (or something else because nothing appears on the screen). Then I flashed the chinese firmware that I used successfully the first time… but it didn't work. I had made a backup of the partitions (before installing the C67ML) with rkflashkit. I flashed them one by one, but it didn't work either. Since then, nothing has worked.
The problem is that I have absolutely no clues about what's going on inside (nothing on the screen, no log files, etc.)