To set up Firefox on your wife's computer with only her settings, you need to create a separate Firefox profile for her. Here's how you can do it:

1. **Open the Firefox Profile Manager**:

   - Close Firefox completely.
   - Press `Windows + R` to open the Run dialog box.
   - Type `firefox.exe -P` (on Mac, type `firefox-bin -P`) and press Enter.

2. **Create a New Profile**:

   - In the Profile Manager, click on "Create Profile..."
   - Follow the prompts to name the new profile (e.g., "Wife's Profile") and click "Finish."

3. **Set Up the New Profile**:

   - Back in the Profile Manager, select the new profile you just created and click "Start Firefox."
   - Firefox will open as if it's the first time, allowing your wife to set it up with her settings, passwords, and logins.

4. **Switch Between Profiles**:

   - Whenever you want to switch between profiles, you can access the Profile Manager again by following the steps above and selecting the desired profile.

For more detailed instructions and to learn more about managing Firefox profiles, [visit here]([edited]).

By creating a separate profile, your wife will have her own personalized Firefox setup without any interference from your settings.

Make sure that you do not connect your wife's device to the same Sync account as used by you as that merges data from both computers (i.e. your wive gets your data and you get your wife's data).