What makes Firefox and Chrome and different in that Firefox needs restarting for most addons whereas Chrome does not? What are the differences between restartess addons and the other ones?
Depends on how the browser is build. In order to change/overwrite files they may need or not to be released (not in use).
By restarting the browser, the files are no longer in use and can be re-written. It may also happen if a cache is needed and cannot be written while in use.
Quoting a Mozilla Dev, Mr. Dave Townsend:
When running Chrome, if you check your task manager you will notice several chrome.exe instances. Chrome uses several modules at once, therefore to reload the addons it wont need to restart totally. In Firefox's case, all you get as extra processes is the maintenance server for errors and the plugin-container process which allows flash content caching and processing among other features.