By default, Firefox automatically allocates
50MB for its disk cache and stores it in the “cache” directory located in the same drive where the browser is installed. Users can view the cache directory location by typing
about:config in the Firefox address bar. Disk cache is a mechanism for improving the time it takes to read from or write to a hard disk. If the browser’s disk cache is located on the same drive where the browser is installed, the browser’s performance technically will be affected.
The performance slowdown issue will be more obvious if Firefox is installed on a Solid State Disk, which is not suitable for write operation. The disk can do read date pretty fast but is overall slow in writing to it. Hence, it is good to relocate the disk cache in Firefox to separate the drives for better performance as well as for security reasons.
The Cache location can be changed by adding the “
browser.cache.disk.parent_directory” preference and entering a folder path. To do it, type
about:config in the Firefox address bar.
Right click your mouse and select New String from the Menu. In the dialog box, just type in
browser.cache.disk.parent_directory as the preference name and the new path to the cache location, for instance
d:\temp. Once this preference is specified, a new directory named “Cache” will be created at the path you set.
Source
Tutorial: How to Move the Firefox Disk Cache To Another Drive
Re: Tutorial: How to Move the Firefox Disk Cache To Another Drive
I just found articles and posted it here with original source. 
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