[Bug]: Running balance text selection partially broken

### What happened?

Normally I like to update investment account growths by:

1. Selecting the previous value in the Running Balance column on the account page and copying it
2. Entering "[current value] - [copied previous value]"
3. Pressing enter to calculate the growth

But recently (maybe a few weeks ago?) the text selection stopped working in Firefox. Happens on my self-hosted instance and also on the demo budget. I also tested on Ungoogled Chromium and it works fine on there. Clip below is in Firefox:

https://github.com/user-attachments/assets/d399f704-9235-495e-8035-112a0f533fed

### How can we reproduce the issue?

1. Go to https://demo.actualbudget.org
2. Open a specific account
3. Menu > Show running balance
4. Try to drag the mouse over the running balance figures

~It will be fine for scheduled transactions rows at the top (in italic), but not for regular rows.~ Update - it's not consistent, in my experience.

### Where are you hosting Actual?

Pikapods

### What browsers are you seeing the problem on?

Firefox

### Operating System

Linux