Automatically set Firefox cache to local temp directory

I think Firefox portable would be a lot faster if it had the option to use a directory of the current computer for the cache, for example, %TEMP%\FirefoxPortable. This folder would be deleted after Firefox was closed.
I know i can add browser.cache.disk.parent_directory to about:config, but since the path has to to have double backslashes for each backslash (ie. C:\Temp\Cache has to be inserted as C:\\Temp\\Cache), i can't use the %TEMP% windows environment variable to set browser.cache.disk.parent_directory because the %TEMP% variable contains single backslashes (ie. C:\Windows\Temp instead of the needed C:\\Windows\\Temp).
Thus, currently only way to use a cache in a local computer is to change browser.cache.disk.parent_directory every time one changes computers.