Using the Hausa “hooked letters” (ɓ Ɓ ɗ Ɗ ƙ Ƙ) with my web browser (Firefox) is no longer a problem for me since I have installed abcTajpu. This addon, which was originally developed for the European Day of Languages 2005, offers a number of ways to enter the non-ASCII letters from various European (and some other) languages. In addition to that, you can define key sequences yourself, e.g. I have defined the following sequences:
Now, whenever I need one of the hooked letters, all I have to do is type the sequence, followed by a function key (I have chosen F2) and — presto-chango — the hooked letter will appear. Pretty cool!