Clearing browsing history in Firefox shouldn't affect files that you saved to the hard drive.

How are you clearing this data ? You aren't using external software to do cleanup ?

Just in case this is a problem with creating the file in the first place, please double-check that the HTML file was saved when using Save Page As before you clear data. You can use the downloads arrow on the toolbar and the folder icon to "Open Containing Folder" and the file should be selected.

Better late than never: I found that it has something to do with "uBlock Origin". Disabling this extension "solved" the issue.

I never noticed a problem with the quality of the saved files: creating a copy before clearing the browsing data was my workaround.

I just now upgraded to Windows 10 and, thus, reinstalled Firefox. Now, the files are not deleted anymore. However, the download is still marked as "failed". Again, I do not see anything missing. So I'll leave it at that.

Thank you for your help.

Hi shipman, it sounds like your workflow is working for you, but if you did want to try something different that doesn't require the companion folder, this extension looks interesting:

https://addons.mozilla.org/firefox/addon/single-file/

Interesting, this seems to work much better than the default method (and it does not clash with my ad blocker). Thanks for the tip!