Don't even bother downloading the latest version of X-notifier it's horrible. I just tested the 3.5.23 on the latest Firefox 53.0 (64.bit) and it's working great. Here are some tips you should do after you reinstalled 3.5.23.

Simply open Firefox click tools>addons>Extentions>
Click on the gear symbol> Then Uncheck "Update add-ons Automatically".

Otherwise If you don't Firefox will Automatically install the latest version again.

While your there you might as well go to Advanced>Update>"Check for Updates,But let you choose whether to install them". This will Prevent Firefox from automatically updated the latest version until your ready to install it.