In reply to a couple of the points raised:

1) With regards to the saved file format:

The page source (HTML) and all of the referenced resources are saved in a single file (.html).

External CSS style sheets are converted to internal CSS style sheets. All other textual resources (scripts & frames) are stored as UTF-8 data URIs.

If the page loader is not used, all binary resources (images, fonts, audios, videos, etc) are stored as Base64 data URIs. In this case, if a binary resource is referenced multiple times, a Base64 data URL will be stored for each reference.

If the page loader is used, all binary resources (images, fonts, audios, videos, etc) are stored as Base64 strings in the page loader script, and are converted to blob URLs when the save page is opened. In this case, if a binary resource is referenced multiple times, its Base 64 string will be stored only once, resulting in much smaller saved files.

2) "When I reopened that page ... most of the images were lacking and much of the layout was somewhat off."

The reason for this discrepancy is that, when you saved the page with Save Page WE, you had the 'Use page loader to reduce file size' option enabled, which means all of the binary resources are represented as blobs.

Before using the Firefox 'Save Page As', you need to use the 'Remove Page Loader' menu item that is built into Save Page WE. Alternatively, you could disable the 'Use page loader to reduce file size' option before saving the page with Save Page WE. Either way, you should find that the re-saved page is pretty much identical to the original page.