You can try this rule as your first rule:
RewriteEngine On
RewriteCond %{THE_REQUEST} \s/+([^?]+)\?folder=([^\s&]+) [NC]
RewriteRule ^ /%2/%1? [R=301,L,NE]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/(.+?)/?$ /$2/?folder=$1 [QSA,L]