Clear SharePoint Designer cache

Sometimes SharePoint Designer loses touch with reality - it demands to check out files that are not checked in, refuses to check in other files and generally misbehaves. This demeanor is sometimes accompanied by this error message:

    "Cannot perform this operation. The file is no longer checked out or has been deleted."

Simply put, SharePoint Designer is out of sync with SharePoint and you have to delete its cache in order to rebuild it. The cache is composed of these 2 folders:

    %APPDATA%\Microsoft\Web Server Extensions\Cache
    %USERPROFILE%\AppData\Local\Microsoft\WebsiteCache

Just delete their contents and you are done.
I found the solution here.

Move/set Firefox cache location

You can do this by creating a new hidden preference.

Type about:config into the location bar and press enter

Accept the warning message that appears, you will be taken to a list of preferences

Right-click somewhere in the list and select "New > String"

For the name of the preference type browser.cache.disk.parent_directory

For its value type the path to where you want to store the cache, for example d:\fftemp

Next locate the preference browser.cache.disk.enable, it must be set to true, if it is not, double-click on it to change its value

Set the size of the cache to something like 32000 to 512000 by setting browser.cache.disk.capacity (32-512MB)

For more information on this see: https://support.mozilla.org/en-US/questions/768867