https://support.mozilla.org/en-US/questions/1066093

The problem could also be caused by an antivirus program if you have one set to scan incoming attachments (disabling antivirus email scanning is recommended, anyway).

with Proximus mail, no problem

Reading messages and downloading attachments through webmail (e.g. Proximus), uses a different set of technologies than doing it with a program like TB. That is why the suggested remedies for your problem apply to TB, but are not relevant to webmail. However, if Proximus accomplishes what you want from email, I see no reason to abandon it.