Do you notice any other settings changing after you exit and start Firefox up again?

Some things to check:

• This support article describes a number of possible reasons for changes to get lost between sessions: How to fix preferences that won't save.

• Some utility programs like Advanced SystemCare have a feature to revert your settings to an earlier state between sessions ("Surfing Protection"). That's another thing to check for if you keep losing your settings changes.

• If you start up Firefox in a sandbox environment (for example, using Sandboxie), changes you make may not be stored on disk, or may be cleared from temporary storage at preset intervals.

No, like i see there is only theme that wont save! i have windows 8.1 on my gf-s laptop is everything ok but she is using win7.

When you come back to Firefox, is the theme listed on the Add-ons page, Appearance section, but it's disabled? Or is it completely deleted?

it's completely deleted

Are you talking about a complete (full) theme or about a Persona (lightweight theme) that only gives a background to the user interface (toolbar) area?

toolbar area

The data of all installed Personas and its background (attention) color and text color are stored in the lightweightThemes.usedThemes pref in JSON object format ({"id":"#","name":"xxx",...}). The first entry of that pref is the currently selected Persona. You may want to keep a backup copy of the current value of the lightweightThemes.usedTheme pref and bookmark the pages of installed Personas on the Add-ons site. The images of the currently selected Persona are stored in the lightweighttheme-header and lightweighttheme-footer files in the Firefox profile folder, so make sure to keep these files as well.

Firefox copies the image files to the lwtheme folder in the Firefox profile folder and sets some prefs to true. lightweightThemes.isThemeSelected = true lightweightThemes.persisted.footerURL = true lightweightThemes.persisted.headerURL = true

ok i manage to solve something and dont know if that have to do with anything but i just turned off some options in Adblock. is it possible because of that addon my theme didnt save?

ok i think what i did solved the problem! so tnx for support!