If that company logo image etc is to a file which is not stored on your computer then it is in effect remote content.

On computer, I have a folder in 'Documents' called 'TBSignatures' It contains all images and html files for various signatures.

Is the company logo an image file stored on your computer? Did you create the signature using Thunderbird Write composing window? If yes: When 'Insert' > 'Image' to insert the logo, did you select the checkbox 'attach this image to the message' - if no then you have only a link to a file which will not display the image. Did you save the signature as a html file to a folder on your computer? If yes: Did you then set up Account Settings for account name to 'attach the signature from a file instead' and then click on 'Choose' and select the relevant html file stored on computer?

See https://support.mozilla.org/en-US/questions/1435746

The file wasn't blocked but there was a problem with the path. Windows in the Dutch version uses "Gebruikers" (displayed in Windows Explorer) for "Users", however under the hood (in the TB link) you have to use "Users" in the path instead of the "Gebruikers".