This is something you can change from your options menu, in the upper right click the button with three horizontal lines and select options. From there make sure you are under the General category on the left and then scroll down to the applications section. In this section you can chose the default action for opening file types, click on portable document format (.pdf) and if foxit is installed on the computer you should have it listed here. Once you change that foxit should pop up for you again.
Foxit isn't a firefox software and install problem you should direct that to foxit support. Also could be a 32 bit to 64 bit firefox install that could also remove previous unverified addons from running on the new install.
Here, maybe this will help. The file that presents in the reader as an image is 1.4 MB. The file that presents as text is only 59 kB. So it would seem the text selectable PDF file is not an image at all. It contains text with font descriptions and location information.
Foxit Reader is one of the best PDF reader out there. Foxit Reader is free to use and it has many premium features that you can unlock buy buying Foxit Reader. In this article, I will show you how to install Foxit Reader on Ubuntu 18.04 LTS.
This is a proprietary app that uses its own installer. I suggest you remove the whole /opt/foxitsoftware/ dir and remove anything in the .local/share/Foxit dir. They should have provided an uninstall tool.
Talos has identified four use-after-free vulnerabilities in Foxit Reader. The reader includes Javascript support to enable dynamic documents and multimedia content, which can be viewed interactively. A specially crafted PDF document can trigger the reuse of previously freed memory, which can lead to arbitrary code execution. An attacker needs to trick a user into opening a malicious file to trigger these vulnerabilities.
Users are encouraged to update these affected products as soon as possible: Foxit Reader 12.0.1.12430. Talos tested and confirmed these versions of the reader could be exploited by these vulnerabilities.
4. After opening Foxit Reader for the first time you may be asked to install an update. Ignore that prompt. You may also be asked to set it as the default PDF reader, please say yes to set Foxit reader as the default PDF viewer.