Hi, what method are you using to delete browsing history? For example:

• Open the Library window using "Show All History" or Ctrl+Shift+h
• Display the relevant entries in the list in the right panel by using either the date range selectors in the left column or the small search box at the upper right
• Single click the first item you want to delete and then add to your selection by either Shift+clicking the last item in a range of items, or Ctrl+click individual items (or Ctrl+A to Select All)
• Press the Delete key on the keyboard (or use the right-click context menu)

But about this:

Just would like to, for example, delete a few pages i visited while searching for a product and now getting drowned in ads about them...

If you get ads about some activity from Site A while browsing on unrelated Site B, that usually is linked up using cross-site tracking cookies. Firefox's Enhanced Tracking Protection feature usually blocks those, but if it's not working, you could consider clearing your cookies. It would be best to only clear the problem cookies, but they might be hard to spot in the list opened by the "Manage Data" button on the Options page. Also, if the sites are related and you are logging in, the activity might be recorded in your account and clearing the cookies won't help in that case.

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/profiles before using sync again.

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address bar.

You will find Places Database near the bottom. Press the Verify Integrity button.

When done, copy and post the results here.

https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

You can check for recently installed suspicious or unknown extensions.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can also do a malware scan just to be sure.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware