You can open the about:config page via the location bar and do a search for imesh via the Filter at the top of the about:config page.
You can reset all imesh related prefs that appear bold (user set) via the right-click context menu to their default values.

See also:

• http://kb.mozillazine.org/Preferences_not_saved
• https://support.mozilla.com/kb/Preferences+are+not+saved

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold(user set).
• Preferences can be reset to the default or changed via the right-click context menu.

hi cor-el, thank you for the answer. did exactly that and modified 4 files that came up in the search for imesh in about;config. I closed the tab. (didn't see any option to save or save changes etc. I could see in tools / options that the default home page was reset to firefox but when I close down the browser and try to reopen firefox the imesh comes right back ! Is imesh legitiamate. Feels like a leech to me !