Yes, Bushfiva, I think I did have the latest and most unadorned copies. The warning was a credible one. I visited the Foxit homepage and read that they had been dealing with issues but their last fix was more than a month before the warning from Mozilla. Well, I call it a mere warning but normally such a warning is to draw attention to the need to update. This time there was no update available via Mozilla, only a very brief explanation of what was claimed to be the issue. Now it is no longer offered as a plugin at all.
I could have reinstalled the latest version from the Foxit homepage (China?) but chose to come cap in hand for advice about Foxit and the best alternatives on offer. Are there any further thoughts or suggestions?