Hi Megan Gleeson:
What is your Dell computer model, Windows operating system [if Win 10 or Win 11 please include the version and build shown at Settings | System | About | Windows Specifications, or open a Run dialog box (Windows key + R) and enter winver], and default browser? Does your Dell computer model have a touch screen or built-in keyboard with a touchpad that can use finger gestures to zoom in and out? Please note that you've posted your question in the Windows Vista board, and support for this operating system ended on 11-Apr-2017. If you do not have a Vista OS we'll ask on of the Forum Moderators to move your question to the appropriate board of this forum.
Try re-booting your computer to see if your display zoom returns to its default setting. If that doesn't help and you have a Win 10 OS the Lifewire article How to Use Windows 10 Display Scaling explains how to access your display settings and adjust the % scaling of your display [Start button | Settings (gear icon) | System | Display | Scale and Layout]. From my Dell Inspiron 15 5584 / Win 10 Pro laptop, which has 15.6" (non-touch) screen:
Dell Inspiron 15 5584 * 64-bit Win 10 Pro v21H2 build 19044.1706 * Firefox v101.0.1 * Microsoft Defender v4.18.2203.5-1.1.19200.5 * Malwarebytes Premium v4.5.9.198-1.0.1689 * Macrium Reflect Free v8.0.6758