ePub creator allows you to save web pages opened in the browser as offline ePub e-books. It currently supports:

any web page that can be opened in Firefox's Reader Mode, which should work for most text-based websites
books opened in the online reader of the library service overdrive.com, which cooperates with many local libraries worldwide

How to create/save books:

open the web page or book you want to save
- on overdrive, go to "Loans" and choose "Read now in browser"
click the extensions icon (blue book with green arrow, should be at the top right of the browser, see screenshot)
wait while the animation on the icon is spinning
- this can take a while if the e-book contains many pictures etc.
- for Reader Mode pages, it will prompt for the books author
save or open the e-book when prompted

Whether saving content with this extension is legal or not depends on the content and your local legislation. Checking that is your own responsibility. Just because you can do it doesn't mean you should. What you get & Troubleshooting:

For general web pages, Mozilla's Readability.js is used to extract the content. Since that is the same software component used by Firefox to generate the Reader view, the result should generally be the same as well. All resources (e.g. images) that show up there should also be included in the ePub book. If parts of the website are missing in the book, please check whether they show up in the reader mode. If not, there is little I can change about that.
Books from overdrive.com are not simply downloaded, but the content is parsed from the open book. The ToC is rewritten, most of the formatting is stripped and everything gets repacked. While images are generally still included, their sizing might be off on some readers, rendering this quite useless for comics and the like.
If you have a problem that is not explained by the limitations above, please check for and/or open an Issue on GutHub.

Permissions used:

Display notifications: Only as direct consequence of clicks on the icon or to report errors
Access your data for all websites: Download page content if you choose so
Access recently closed tabs: Remove own popups from the recently closed windows list