Consolidate /b/ redirects for /firefox/ URLs

[Bug 1074261](https://bugzilla.mozilla.org/show_bug.cgi?id=1074261)

The following lines may also be able to be removed, but I wasn't sure. Needs discussion.

```apache
RewriteRule ^/(\w{2,3}(?:-\w{2})?/)?(firefox/)?dnt(.*)$ /b/$1$2dnt$3 [PT]

RewriteRule ^/(\w{2,3}(?:-\w{2})?/)?firefox(/(?:\d+\.\d+\.?(?:\d+)?\.?(?:\d+)?(?:[a|b]?)(?:\d*)(?:pre)?(?:\d)?))?/whatsnew(/?)$ /b/$1firefox$2/whatsnew$3 [PT]

RewriteRule ^/(\w{2,3}(?:-\w{2})?/)?(firefox|mobile)/((?:beta|aurora|organizations)/)?notes(/?)$ /b/$1$2/$3notes$4 [PT]

RewriteRule ^/(\w{2,3}(?:-\w{2})?/)?firefox(/(?:\d+\.\d+\.?(?:\d+)?\.?(?:\d+)?(?:[a|b]?)(?:\d*)(?:pre)?(?:\d)?))?/firstrun(/?)$ /b/$1firefox$2/firstrun$3 [PT]

RewriteRule ^/(\w{2,3}(?:-\w{2}(?:-mac)?)?/)?(firefox|mobile)/(2[38]\.0(?:\.\d)?)/releasenotes(/)?$ /b/$1$2/$3/releasenotes$4 [L,PT]

RewriteRule ^/(\w{2,3}(?:-\w{2}(?:-mac)?)?/)?(firefox|mobile)/(29\.0(?:beta|\.\d)?)/releasenotes(/)?$ /b/$1$2/$3/releasenotes$4 [L,PT]

RewriteRule ^/(\w{2,3}(?:-\w{2}(?:-mac)?)?/)?(firefox|mobile)/([3-9]\d\.\d(?:a2|beta|\.\d)?)/(aurora|release)notes(/)?$ /b/$1$2/$3/$4notes$5 [L,PT]

RewriteRule ^/(\w{2,3}(?:-\w{2}(?:-mac)?)?/)?firefox/(24\.[5678]\.\d)/releasenotes(/)?$ /b/$1firefox/$2/releasenotes$3 [L,PT]

RewriteRule ^/(\w{2,3}(?:-\w{2})?/)?firefox(/(?:\d+\.\d+\.?(?:\d+)?\.?(?:\d+)?(?:[a|b]?)(?:\d*)(?:pre)?(?:\d)?))?/tour(/?)$ /b/$1firefox$2/tour$3 [PT]
```
While testing this I found that https://www.mozilla.org/en-US/firefox/memory/ is currently a 404-- the bug indicates that this was added for a new page, but I'm not sure if that page actually ever got created. Not an issue at all for this PR; just commentary.