If you have issues, you'll have to search for the directories where your plug-ins are housed, and give read or read/write access permissions in the firefox-sandbox file. There is only write permission to the ~/Downloads directory, so if you want to save files in a different location, you will have to change the firefox-sandbox file or move them after the download has finished. First, create the following file and save it somewhere as firefox-sandbox:
;; http://codereview.chromium.org/379019/diff/1/2 (version 1) (deny default) (allow file-write* file-read-data file-read-metadata (regex "^/Users/user_name/Downloads") (regex "^/Users/user_name/Library/Application Support/Mozilla") (regex "^/Users/user_name/Library/Application Support/Firefox") (regex "^/Users/user_name/Library/Preferences") (regex "^/Users/user_name/Library/PreferencePanes") (regex "^/Users/user_name/Library/Caches/Firefox") (regex "^/Users/user_name/Library/Caches/TemporaryItems") (regex "^/Applications/Firefox.app") (regex "^(/private)?/tmp/")) (allow file-read-data file-read-metadata (regex "^/dev/autofs.*") (regex "^/Library/Preferences") (regex "^/Library/Internet Plug-Ins") (regex "^/Library/PreferencePanes") (regex "^/usr/share/icu") (regex "^/usr/share/locale") (regex "^/System/Library") (regex "^/Applications/Firefox.app") (regex "^/usr/lib") (regex "^/var") (regex #"Frameworks/SDL.framework") ; Our Module Directory Services cache (regex "^/private/var/tmp/mds/") (regex "^/private/var/tmp/mds/[0-9]+(/|$)") (regex "^/Users/user_name")) (allow mach* sysctl-read) (import "/usr/share/sandbox/bsd.sb") (deny file-write-data (regex #"^(/private)?/etc/localtime$" #"^/usr/share/nls/" #"^/usr/share/zoneinfo/")) (allow process-exec (regex "^/Applications/Firefox.app")) (allow network*)
[robg adds: I haven't tested this one.]

