What other technology than flash could block changing editing webpage via e.g. firebug in webbrowser?
Because everybody can change the look of the webpages and inject javascript etc. to the website.
To my knowledge only flash and silverlight can prevent this behaviour, but I might be wrong.
Do you know any other technologies used nowadays that are “uneditable” like flash or silverlight?
Any changes made by the user with Firebug or similar will only affect their view of the page, and will disappear as soon as they reload the page. It is not in any way a safety hazard, so just forget about it.
The only thing you should care about is validating whatever the user sends to the server, such as making sure numbers are indeed numbers, email addresses are email addresses, and so on.