Thanks for your tips. My answer to all the questions follows:
1) I did not make any changes in the repository.
2) I did not download Firefox update manually.
3) It does not happen with the live CD.
And strangely enough, the problem has gone away today,
Could it be the weather conditions?
P.S. Any tinkering with the system is done on a separate 20 GB disc which I use for testing. If something passes the test, I may consider it as a candidate for a main disc installation.
What I have done lately is the installation of Konqueror on BEA (very useful as a control panel and file browser)
In addition, I have the kicker working right above the bottom Gnome panel.
So, I have the K menu besides the LinuxMint main menu. ( I have done away with the verbal Gnome menu)
They say 'variety is the spice of life'
Best regards to all,