I have never registered a Firefox account, but I have been using the plug-in you wrote for a year. The "save webpage" function that comes with Firefox or Google Chrome is very ineffective and will export a bunch of code with unclear meaning, while your plug-in will export a complete HTML document.
Some websites prohibit copying of its content, use your plugin to save first and then copy, this works great!

Good

How can I find the directory where the files are saved by default?

我觉得你的拓展实在是太棒了(｡･ω･｡)ﾉ♡

How do you make it so it asks you where to save it everytime?

This functionality should be provided by browsers as a default. They don't, so this app fills the gap.
Functions as expected. Thanks!

First thing that has worked to save a page that had two frames with tables in it!

i used it for 3 years. very usefull to save documentation or to share information.