Profile data can be removed or not recorded several ways;

• 1) Running Firefox in Private Mode (nothing is recorded)
• 2) Privacy settings; (Type about:preferences#privacy<enter> in the address bar.)
• 3) Many disk cleaners and anti-virus type programs can remove data from browsers. Open their settings and have them leave the browsers alone.
• 4) Mal-ware.

FredMcD said

Profile data can be removed or not recorded several ways;

• 1) Running Firefox in Private Mode (nothing is recorded)
• 2) Privacy settings; (Type about:preferences#privacy<enter> in the address bar.)
• 3) Many disk cleaners and anti-virus type programs can remove data from browsers. Open their settings and have them leave the browsers alone.
• 4) Mal-ware.

1. I dont run Firefox in Private Mode, never. 2. I tweak the Privacy Setting (remember history, customs setting...) few times but I doesn't help. 3. I use the same anti-virus programs for few years, it never delete my history, just recently, history keep dissappearing. I dont use disk cleaner. 4. I have anti mal-ware program, scan every month.

I thought the problem was with sync, so I turn off the sync, doesnt help. Try to re-installed Firefox yesterday, lets see if it's help.

If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Once the problem is fixed, perform the same repair on all computers before using sync again.