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Fix: Firefox 35 can’t install add-ons and extensions

Firefox just updated itself to version 35 on my current PC. I tried to install a new add-on (Session Manager, in case you are curious) but it failed to install for no reason. It gave the error message Add-on could not be installed because Firefox cannot modify the needed file.
Add-on could not be installed Firefox 35
Here is how you can resolve this issue if it ever happens on your PC.

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