For starters, Thunderbird is not Firefox. Firefox is a web browser, Thunderbird is an email client.
Second, this is a very common problem when people try and embed photos in emails as opposed to attach them. The file pointer in the email is on your specific system, so unless every recipient has that file in exactly the same spot, it doesn't work. This isn't a Thunderbird specific problem, plenty of people using other email clients have the same issue. Attach the photos, don't try and embed them.
For that matter, just forget all about any kind of special formatting in email, it doesn't work. Email was designed for plain text only. Any time you try and go beyond that you increase the odds of people not being able to receive your message properly. You want to share photos with someone, open an account on a photo sharing site like Flickr or Google's Picasa. Upload the photos there, provide a link in an email, let your email recipients go look at them at their leisure instead of forcing them to download large image files. Not to mention web servers tend to have significantly more processing power AND bandwidth, which makes them infinitely better suited than email servers when it comes to sharing large files like photos.
We all have to realize that we are part of a community on the Internet. The actions of a single person can have a negative effect on countless others. It would be irresponsible to be concerned purely with our own wants and desires. If everyone tries to act in their own self-interest, then everyone loses. It's sort of like the economy these days. Businesses don't want to start ramping up production because consumers aren't buying, consumers aren't buying because they need to try and conserve money. Money they might have if businesses started producing more and hiring people. Every company and person is acting in their own self-interest, and it's working against the common good we all desire. You sending a bunch of large photos in an email may mean that someone else's important email (and we'll ignore the fact that email shouldn't be trusted for important communications) may not go through or it gets stuck in a server somewhere, waiting on your message to clear before it can go on its way.
Anyway, this concludes today's Internet Civics 101 class.