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Recent Firefox browser versions have seen issue when uploading files to SDL TMS (below article):
To confirm the engineering team will take the latest available browsers at the time of development and test SDL TMS against these. These become the supported browser versions as stated in SDL TMS - supported browsers.
Some browsers may automatically update to the latest version - normally SDL TMS will be unaffected by this and function as expected - unfortunately this release of Firefox includes a change that does.