I called for more help.

We need to look at the sync logs.

Type about:sync-log<enter> in the address bar.

Open the last file created and Copy its contents Now Paste it into this forum.

Here is the file you requested.

Index of file:///C:/Users/Sylvie/AppData/Roaming/Mozilla/Firefox/Profiles/abf8k7cy.default-1519600237525/weave/logs/

Up to higher level directory Name Size Last Modified File:error-sync-1526179616815.txt 4 KB 5/12/2018 10:46:56 PM File:error-sync-1526215123890.txt 4 KB 5/13/2018 8:38:44 AM File:error-sync-1526216079411.txt 3 KB 5/13/2018 8:54:39 AM File:error-sync-1526695145304.txt 4 KB 5/18/2018 9:59:05 PM File:error-sync-1526696066427.txt 4 KB 5/18/2018 10:14:26 PM File:error-sync-1526698035892.txt 4 KB 5/18/2018 10:47:15 PM File:error-sync-1526711256402.txt 4 KB 5/19/2018 2:27:36 AM File:error-sync-1526763619213.txt 4 KB 5/19/2018 5:00:19 PM File:error-sync-1526827186528.txt 4 KB 5/20/2018 10:39:46 AM File:error-sync-1526941789889.txt 4 KB 5/21/2018 6:29:50 PM File:error-sync-1526942694872.txt 22 KB 5/21/2018 6:44:55 PM File:error-sync-1526943017454.txt 4 KB 5/21/2018 6:50:17 PM File:error-sync-1526943050474.txt 4 KB 5/21/2018 6:50:50 PM File:error-sync-1526987027023.txt 4 KB 5/22/2018 7:03:47 AM File:error-sync-1527018848444.txt 4 KB 5/22/2018 3:54:09 PM

FredMcD said

Type about:sync-log<enter> in the address bar.

Open the last file created and Copy its contents
Now Paste it into this forum.