Hello,

The start-up issue is a Firefox bug - due to asynchronous nature of loading add-ons sometimes the first tab is loaded before the add-on is loaded which can result in blank page, wrongly loaded page or default new tab page.

Regarding the local .html files - this is now completely blocked by Firefox internal security for all add-ons. You can create such links in my speed dial, but Firefox won't allow you to click on them.
There was a discussion about this here:
https://discourse.mozilla.org/t/opening-links-to-local-files-file/16449
and some official text here:
https://wiki.mozilla.org/WebExtensions/Filesystem

Situation in Chrome browser is very similar but Chrome added option (checkbox) to enable access to local files for specific add-on on their add-on page.
I hope something similar will come to Firefox one day, but until then, there is nothing I can do about it.

Best regards,
Juraj Mäsiar