This problem may sound weird, but strange signs appear in my inbox, sent mails and spam folders. (A picture says more than thousands words, so see below for an example). The signs seems somehow to be related with the content of the messages (e.g. if the message is a spam mail related to shopping, there will be a shopping chart) I'm using Thunderbird 52.8.0 on a Ubuntu 18.04 machine. This problem appears since I upgraded from Ubuntu 17.04 to 18.04 (which perhaps included an update of Thunderbird).

Any help is very much appreciated!