`$2` won't be anything here. The `(?:...)` parenthetical style makes it not capture the match. I'm guessing you really don't care what the end is and just want to redirect. Probably best to just do:

```apache
RewriteRule ^/(\w{2,3}(?:-\w{2}(?:-mac)?)?/)?mobile/customize(?:/.*)?$ /$1firefox/mobile/features/ [L,R=301] 
```