I am experiencing no problems at all with the builder this morning which would lead to the assumption that your browser is at fault
- What are you using, Chrome is best for the builder (may be the only choice these days, used to be Firefox)
- Is your chrome up to date
- Are you running lots of extensions in your browser (plugins) that could cause conflicts
- Have you done a total clear down of your cache
All basic stuff, but I can assure you as a fellow user the builder is working fine as I have already updated and downloaded two extensions this morning with no issues