Do you download and save the file to edit the file or do you have a local copy?

What editor are you using to make the edits?

Can't this editor handle HTML files?

If you are using Windows' Notepad application as your editor, consider switching to the free, open source program Notepad++. It has syntax highlighting so that it will be much easier to distinguish tags from content, and it also helps with seeing typos in scripts.

http://notepad-plus-plus.org/

Now... your default browser will still launch .html files if you double-click them, but you can right-click the file an "Edit with Notepad++" should be an option.

(To get around to your original question, Firefox does not make a good file editor.)