You can upgrade to any "patch" FW. Security FW is full FW releases with just immediate security fixes. They may or may not included any feature fixes. Mainly just security fixes.
2. WW is for World Wide ... WW is safe to use.
3. Yes, please use the most recent version of FW.
Just confirming,
Aus firmware is 1.01b90 which is WAY out of date.
http://support.dlink.com.au/Download/download.aspx?product=DIR-880L&type=Firmware
Main site shows 1.04b01 WW
http://support.dlink.com/ProductInfo.aspx?m=DIR-880L
DIR-880_REVA_FIRMWARE_PATCH_1.04.B01_WW.ZIP
Providing I can un-brick my 880L (suggestions welcomed, it's deader than a flat cat on a road), I can flash to 1.04b01 WW ?