[Bug Report] Textarea autosize does not work in Firefox

### Element UI version
2.15.14

### OS/Browsers version
Ubuntu/Firefox119

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/r9Lcpwf6/1/

### Steps to reproduce
Hi, this is a more-less request for support from a newbie :)

Use the attached jsfiddle to reproduce the issue

### What is Expected?
Textarea autosize works in Firefox

### What is actually happening?
Textarea autosize does not count the number of rows correctrly