The new Firefox brings to the table a huge list of fixes, but there are still some problems that could bother some users; and the developer is well aware of them and trying to work them out.
First on the list is a crash that occurs when the web browser is started using a locked profile; luckily, not too many users get to experience this behavior as most of them work with a single profile. Also on the list is an issue with slow scrolling in the main window of Gmail for some users.
Both of them have turned into somewhat permanent problems because they date as far back as Firefox 4.0. But, as per the last comments on the bug’s discussion page, the latter should not have made it on the known issues list for Firefox 19.
If you have problems using MooTools 1.2.x, you should update it at least to version 1.3, which is compatible with Firefox 18 and above.
The developer is not expecting additional problems coming their way, since the PDF.js component is the only major feature in Firefox 19 and they already have a hotfix add-on prepared, just in case the feature needs to be turned off.